"I unconditionally condemn today's murder of two police officers in New York City. Two brave men won't be going home to their loved ones tonight, and for that, there is no justification," Obama said in a statement following the Saturday murders. "Tonight, I ask people to reject violence and words that harm, and turn to words that heal."
A gunman assassinated two uniformed New York police officers in a cold-blooded attack following weeks of outrage over police killings of unarmed black men.
The two officers, one a newlywed, were shot in the head through the window of their patrol car in broad daylight in Brooklyn in an attack that left New York reeling just days before Christmas.
Police named the shooter as 28-year-old Ismaaiyl Brinsley, who shot and seriously wounded his ex-girlfriend in Baltimore early on Saturday before driving to Brooklyn to murder the two officers.
Cities across the US have had weeks of protests condemning a string of recent police killings of unarmed black men and decisions by grand juries not to press charges against the white officers responsible.
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New York Police Commissioner Bill Bratton said social media postings showed that Brinsley "had a very strong bias against police officers".
"Today, two of New York's finest were shot and killed with no warning, no provocation," said an emotional Bratton.
"They were quite simply assassinated."
Wenjian Liu, a seven-year police veteran who got married two months ago, and Rafael Ramos, who leaves behind a 13-year-old son, were "ambushed and murdered" as they sat in the front seat of a marked NYPD police car, officials said.
Neither officer had the opportunity to draw their weapons before Brinsley opened fire with several rounds and fled to a nearby subway station.
He shot himself in the head on the platform, where Bratton said a silver semi-automatic firearm was recovered near his body.
New York Mayor Bill de Blasio said the city of 8.4 million was in mourning.
"When a police officer is murdered, it tears at the foundation of our society. It is an attack on all of us," he said.
"Our entire city was attacked by this heinous individual."
It is the seventh time since 1972 that New York police partners have been murdered together, according to Bratton.
The police chief said the motive was still under investigation, but did not rule out a connection to anti-police protests that have swept the country.
"One of the unfortunate aspects at times is some people get caught up in these and go in directions they should not," he said.
In a cruel twist of fate, Bratton said Baltimore police warned New York colleagues that Brinsley may be on the loose just as the murder took place.
Brinsley is accused of boasting on Instagram just hours before the attack that he would kill police officers.
"They Take 1 Of Ours ... Let's Take 2 of Theirs," read a comment next to a photo of a silver handgun, referencing the deaths at the hands of police of Eric Garner and Michael Brown.
Garner, an unarmed father of six, died after police held him in a chokehold while he was being arrested for selling illegal cigarettes in New York in July.
Brown, an 18-year-old in the Ferguson suburb of St Louis, Missouri, was shot dead by a police officer in August, sparking months of protests.
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