U.S. President Barack Obama (bottom) addresses lawmakers and family members of slain police officers before 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 Justin Ramos (L-R), Vice President Joe Biden, Jayden Ramos, Maritza Ramos, Senator Ben Cardin (D-MD), Representative Dave Reichert (R-WA), Representative Bill Pascrell (D-NJ) and Wei Tang Liu. REUTERS/Jonathan Ernst