U.S. President Barack Obama (C) hugs Maritza Ramos, wife of slain New York City police officer Rafael Ramos, after signing the Rafael Ramos and Wenjian Liu National Blue Alert Act into law in the Oval Office in Washington May 19, 2015. Ramos and Liu were the New York City police officers shot dead while in a patrol car last December by a killer who said he wanted to kill police in revenge for deaths of black men by police. Also pictured are Vice President Joe Biden (L-R), Justin Ramos, Jayden Ramos, Senator Ben Cardin (D-MD) and Representative Dave Reichert (R-WA). REUTERS/Jonathan Ernst