Maltese troops survey a hijacked Libyan Afriqiyah Airways Airbus A320 on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zammit Lupi -MALTA OUT- TPX IMAGES OF THE DAY

Maltese troops survey a hijacked Libyan Afriqiyah Airways Airbus A320 on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zamit-Lupi MALTA OUT 

Maltese troops survey a hijacked Libyan Afriqiyah Airways Airbus A320 on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zamit-Lupi MALTA OUT 

A hijacked Libyan Afriqiyah Airways Airbus A320 stands on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zamit-Lupi MALTA OUT 

Maltese troops survey a hijacked Libyan Afriqiyah Airways Airbus A320 on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zamit-Lupi MALTA OUT 

Maltese troops survey a hijacked Libyan Afriqiyah Airways Airbus A320 on the runway at Malta Airport, December 23, 2016. REUTERS/Darrin Zamit-Lupi MALTA OUT 

Protesters and family members of 9/11 victims protest in front of the White House regarding President Barack Obama's threatened veto of the Justice Against Sponsors of Terrorism Act (JASTA) in Washington, U.S., September 20, 2016. REUTERS/Gary Cameron 

Protesters and family members of 9/11 victims hold placards in front of the White House regarding President Barack Obama's threatened veto of the Justice Against Sponsors of Terrorism Act (JASTA) in Washington, U.S., September 20, 2016. REUTERS/Gary Cameron 

Protesters and family members of 9/11 victims protest in front of the White House regarding President Barack Obama's threatened veto of the Justice Against Sponsors of Terrorism Act (JASTA) in Washington, U.S., September 20, 2016. REUTERS/Gary Cameron 

Visitors look out over the National September 11 Memorial and Museum on the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Lucas Jackson

The Tribute in Light shines on the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly TPX IMAGES OF THE DAY

U.S. Democratic presidential candidate Hillary Clinton leaves ceremonies marking the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

A girl runs out of the crowd to greet U.S. Democratic presidential candidate Hillary Clinton as she leaves her daughter Chelsea's home in New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton and her traveling press secretary Nick Merrill (R) attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9-11 Memorial in New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump and U.S. Secretary of Homeland Security Jeh Johnson attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9-11 Memorial in New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9-11 Memorial in New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump and New Jersey Governor Chris Christie attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9-11 Memorial in New York, United States September 11, 2016. REUTERS/Brian Snyder

Former World Trade Center employee Javier Castro Suarez pauses in the recently completed Oculus at the World Trade Center on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly TPX IMAGES OF THE DAY

A firefighter pauses as he walks through the St. Paul's Chapel cemetery on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

McKenna Thomas of Philadelphia pauses to read memorial ribbons tied to the exterior wall of St. Paul's Chapel on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

A man ties a memorial ribbon to the exterior wall of St. Paul's Chapel on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Firefighters pause on the Brooklyn Bridge during a memorial march on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Members of Engine 219 Ladder 105 and fellow firefighters march to Brooklyn over the Brooklyn Bridge in honor of firefighters lost in the 9/11 attacks on the event's 15th anniversary in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly TPX IMAGES OF THE DAY

A man arrives at the World Trade Center complex on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Reverend Doctor William Lupfer rings the Bell of Hope in Saint Paul's Chapel at 8:46am on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Clark County Firefighter Fred Goff IV pauses by the Bell of Hope at St Paul's Chapel on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Firefighters arrive for a memorial service at the World Trade Center complex on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly TPX IMAGES OF THE DAY

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton gestures to the members of the media gathered across the street from her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

Firefighters arrive for a memorial service at the World Trade Center complex on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton climbs into her van outside her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton climbs into her van outside her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, New York, United States September 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated." REUTERS/Brian Snyder TPX IMAGES OF THE DAY

Police officers stand outside Ground Zero on the morning of the 15th anniversary of the 9/11 attacks in Manhattan, New York, U.S., September 11, 2016. REUTERS/Andrew Kelly 

U.S. Democratic presidential candidate Hillary Clinton leaves ceremonies marking the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder TPX IMAGES OF THE DAY

U.S. Democratic presidential candidate Hillary Clinton leaves ceremonies marking the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton leaves ceremonies marking the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton, New York Mayor Bill de Blasio (R) and U.S. Senator Chuck Schumer (L) attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton, New York Mayor Bill de Blasio (R) and U.S. Senator Chuck Schumer attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton and New York Mayor Bill de Blasio (R) attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump and New Jersey Governor Chris Christie attend ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton arrives for ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump (R) and former New York City Mayor Rudy Giuliani (2nd L) arrive for ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Republican presidential candidate Donald Trump attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

U.S. Democratic presidential candidate Hillary Clinton attends ceremonies to mark the 15th anniversary of the September 11 attacks at the National 9/11 Memorial in New York, New York, United States September 11, 2016. REUTERS/Brian Snyder

A mourner places a flag in the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Lower Manhattan is reflected in the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Day breaks over lower Manhattan on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Day breaks over lower Manhattan on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Steven Monetti Jr. reads names on the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

Steven Monetti Jr. walks through the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly 

People gather at the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, U.S., September 11, 2016. REUTERS/Andrew Kelly