The United States President Barack Obama has strongly condemned the killings of two New York City police officers shot by a man who then killed himself on Saturday.
Mr Obama said the officers would not be going home to their loved ones "and for that, there is no justification".
The two were killed while on patrol in Brooklyn. An official said the two officers shot dead while they were sitting in their patrol car, were "assassinated".
The gunman had posted anti-police messages online, amid continuing tensions over police tactics.
New York mayor Bill de Blasio on Sunday ordered flags flown at half staff around the city. New York City's main police union harshly criticized the city's first Democratic mayor in two decades for being insufficiently supportive of the department during recent waves of anti-police violence.
The gunman, Ismaaiyl Abdula Brinsley, 28, travelled from Baltimore, where police said he had shot and wounded his girlfriend, to New York and during the day posted on the social media service Instagram that he would be "putting wings on pigs today," using an anti-police slur.
The two police officers, Rafael Ramos, 40, and Wenjian Liu, 32, had no time to react when Brinsley appeared next to their vehicle, and shot both officers with a silver semi-automatic handgun, NYPD Police Commissioner William Bratton said.
Black American civil-rights activist, Al Sharpton, has also condemned the killing of the two police officers.
Mr Sharpton who has recently led anti-police protests, said the protesters are not motivated by revenge.
"If we go into an area where it's an eye for an eye, then it is only a matter of who can out pluck eyes, rather than who can make the system fair."
Meanwhile, another police officer in the state of Florida has been killed in a shooting.
It happened early yesterday, the day after the New York City shooting.
Authorities said the gunman had indicated on social media he might seek revenge for recent police killings of unarmed black men.
Yesterday's shooting happened in Tarpon Springs, about 50 kilometres north-west of Tampa, and the police said they had a suspect in custody.
They have not immediately identified the slain officer or suspect, and have not given a motive.