Police-community relations, already strained by nationwide protests over the deaths in recent months of unarmed black people at the hands of white officers, grew even more fragile Sunday in the wake of the assassination-style slayings of two New York City patrolmen.

A day after Officers Wenjian Liu and Rafael Ramos were fatally shot in their patrol car, recriminations flew in the country's largest city, with Mayor Bill de Blasio standing at the epicenter. Critics blamed the mayor and his aggressive campaign to reform police practices for the shootings, with officers taking the extraordinary step of silently turning their backs on him as he entered the hospital where the patrolmen died.

Relations between the mayor and officers have become so strained, former police commissioner Ray Kelly said in an interview, that de Blasio "probably needs an intermediary to go between himself and the [police] unions, maybe a religious leader."

While other New York elected officials defended the mayor, concerns about more anti-police violence extended elsewhere, with departments and police unions in other states warning officers to be on high alert.

More information emerged Sunday about the slayings of the officers, as well as the background of the shooter. Police described Ismaaiyl Brinsley, 28, as a highly troubled man with a violent past, saying that he had been arrested 19 times and that his mother had feared him. A year ago, they said, he tried to hang himself.

While authorities said it was unclear whether Brinsley had been involved in the protests that have roiled the nation in recent weeks, leaders of the burgeoning national movement against police violence worked to distance themselves from the events in New York. Even so, the level of blame laid at their feet by some politicians illustrated the tense atmosphere.

Just three hours before Saturday's shootings, authorities said, Brinsley declared his intention on his Instagram account to kill police officers as retribution for the police killings of Michael Brown in Missouri and Eric Garner in New York City.

Former New York Mayor Rudolph W. Giuliani, known for his tough anti-crime policies in the 1990s, blamed not only the protesters for Saturday's shootings but also the sympathy they have garnered from de Blasio and President Obama.

"We've had four months of propaganda, starting with the president, that everybody should hate the police," Giuliani said on Fox and Friends Weekend. "I don't care how you want to describe it: That's what those protests are all about."

De Blasio has condemned the killings as "a despicable act," and the White House said Obama called New York City Police Commissioner William Bratton on Sunday morning to express his condolences.

"The President reiterated his call for the American people to reject violence and words that harm, and turn to words that heal - prayer, patient dialogue, and sympathy for the friends and family of the fallen," the White House said in a statement.

Police on Sunday said Brinsley had a long criminal record and an apparent history of mental illness.

Born in Brooklyn, Brinsley attended high school in New Jersey, with his mother telling officers that he was violent growing up and that she feared him, according to Robert Boyce, NYPD chief of detectives.

While Boyce said at a news conference that he would not speculate on a motive, he said Brinsley had burned a flag in one Instagram posting and mentioned Brown and Garner in others.

The recriminations directed primarily at de Blasio for the shootings began Saturday, with the head of the New York City police union saying the mayor had "blood on his hands" and former New York Gov. George Pataki saying on Twitter that the shootings "sadly are a predictable outcome of divisive anti-cop rhetoric" of the mayor and Attorney General Eric H. Holder Jr.

Asked about Pataki's tweet in an interview Sunday on CBS's Face the Nation," Sen. Lindsey O. Graham (R., S.C.) said, "I blame the shooter and nobody else."

But he added, "I think the mayor of New York has probably undercut his cops, and the attorney general is trying to walk a fine line."

Others defended de Blasio, arguing that he is trying to bridge divisions, not widen them.

Leaders of the diverse movement that has been protesting police tactics also took pains to distance themselves and their organizations from the killings.