PHILADELPHIA - The fatal police shooting of a 27-year-old Black man in Pennsylvania's largest city brought demands Tuesday for an overhaul of the way American communities respond to mental health crises, as well as appeals for calm after a night of destructive violence. 

The killing of Walter Wallace Jr. on a Philadelphia street Monday afternoon - in full view of residents, including his mother, in an incident recorded on cellphone video - became the latest police shooting to prompt outraged protests in a year that has been regularly punctuated by them.

Thirty police officers were injured and 91 people were arrested overnight Monday amid looting and arson, and authorities were bracing for more potential violence Tuesday night. Protests, led by Black clergy members, that began late Tuesday afternoon at the scene of the shooting were peaceful.

Wallace was armed with a knife when he was shot and was advancing toward a pair of officers who had demanded that he drop the weapon. His family said he suffered from mental illness and angrily questioned why police had not used nonlethal methods to subdue him.

The two officers, who each fired approximately seven shots, have been taken off duty as authorities investigate. Philadelphia District Attorney Larry Krasner said Tuesday afternoon that his office would carefully review the incident before deciding whether charges are warranted.

"We are not out to cover for anybody," he said during a news conference. "And we are not out to get anybody."

Philadelphia officials provided only scant information about the shooting itself, leaving the public with relatively few details about a deadly encounter that has rocked the city.

Mayor Jim Kenney, who said he had spoken to Wallace's parents and wife, acknowledged that the video presented "difficult questions that must be answered." He declined during a media briefing to say what questions were raised for him, saying only that "seeing someone shot is a very upsetting thing to view."

Kenney said he was confident in the Philadelphia police's ability to investigate what happened.

"We need a speedy and transparent resolution for the sake of Mr. Wallace, his family, the officers and for all of Philadelphia," he said during the briefing.

The killing in the swing state of Pennsylvania just over a week before Election Day once again injected issues of racial justice and urban violence into the heart of the presidential campaign. In a statement Tuesday afternoon, Democratic nominee Joe Biden and his running mate, Sen. Kamala Harris of California, decried Wallace's killing.

"We cannot accept that in this country a mental health crisis ends in death," they said. "It makes the shock and grief and violence of yesterday's shooting that much more painful, especially for a community that has already endured so much trauma."

The Democrats also condemned Monday night's rioting, saying that "looting is not a protest, it is a crime."

President Donald Trump, speaking at a campaign rally in Wisconsin on Tuesday evening, sought - without evidence - to tie his opponent to the unrest.

"Last night Philadelphia was torn up by Biden-supporting radicals," Trump said. "Thirty police officers, Philadelphia police officers, they were injured, some badly. Biden stands with the rioters, and I stand with the heroes of law enforcement."

Wallace's death mirrors other recent instances of police violence caught on camera that have fueled a summer and fall of unrest over police brutality, and it raises new questions about how police handle mental health crises.

Wallace was one of at least 804 people shot and killed by police this year as of Monday, according to The Washington Post's police shootings database, which tracks such incidents nationwide. One in five fatal shootings by police this year involved people who were suffering from mental health issues or in the midst of a crisis.

The database figures do not include those who die at police hands by other means, including Daniel Prude, whose family says he was suffering a mental health crisis when officers in Rochester, N.Y., pinned him down and put a hood over his head.

This persistent toll has highlighted concerns about how authorities respond to people suffering mental anguish. It has also raised questions about whether armed police, who may lack the training needed to handle all of these calls, should be responding to those crises.

"Unfortunately this is something that happens all the time. It's been happening day after day, year after year," said Angela Kimball, policy and advocacy director for the National Alliance on Mental Illness. "It is just now that communities are starting to say, 'No, this is wrong.'"

A better solution, she said, is for communities to establish teams of "mental health professionals who are trained at de-escalating. They're not in uniform. They're not shouting. They're able to establish rapport and connect someone to treatment and support."

As protests over policing and racial injustice have gripped American cities this year, the debate over taking funding from law enforcement and shifting it instead to social services has highlighted the issue, with some communities experimenting with alternatives.

On Monday, officials in Los Angeles unveiled a pilot program planned for early next year that would have some mental health workers respond to certain 911 calls that could otherwise go to the police. Michel Moore, the Los Angeles police chief, said the effort meant "pulling things off of our plate and putting them with our mental health professionals" instead.

The incident in Philadelphia began about 4 p.m. Monday when police responded to a report of a man with a knife. A video of the encounter taken by a bystander and posted on social media showed Wallace's mother trying to restrain him, putting her body between the man and the two police officers. He pushed her away, then turned to walk toward the police. The officers backed away but kept their guns pointed at Wallace.

"Put the knife down," one of the officers shouted in the video.

The camera panned away from the scene as officers opened fire, shooting Wallace several times. Wallace was pronounced dead after being taken to a hospital.

A man who identified himself as Brandon and declined to give his last name called Wallace his "little brother" as he waited outside Wallace's parents' home Tuesday. Wallace, he said, "was in and out of [the] hospital since he was a little kid" because of mental health issues. Brandon said that after getting on his medication, Wallace would "be cool."

Sam White, who identified himself as Wallace's cousin, said Tuesday that Wallace had gotten married a few weeks ago.

"He wanted to be a musician, he loved music," said White, 53. He added later, "His dreams are cut down because he got murdered in the street."

White declined to describe his cousin's specific symptoms, saying only: "Mental health is real. Depression is real."

He said Wallace's parents were distraught, describing them as secluded in a brick rowhouse where they were in mourning. He also said they were appalled by the chaos that unfolded overnight Monday and into Tuesday.

Philadelphia Police Commissioner Danielle Outlaw said the officers injured during the unrest mostly sustained cuts and bruises after bricks, rocks and debris struck them. A sergeant was intentionally run over by someone in a pickup truck and her leg was broken, Outlaw said, adding that she was in stable condition Tuesday.

While peaceful protests rippled across the city Monday after the shooting, the "incidents of civil unrest" that followed were "not part of these protests, and it did not serve any legitimate purpose," she said.

Outlaw said the department was still evaluating what information about the shooting would be released and when - including the identities of the officers involved. She declined to commit to releasing the body camera footage, saying authorities were still reviewing what they could make public.

Outlaw said neither officer had a Taser, noting that not every officer in the department has been issued one. The officers had not been interviewed as of Tuesday afternoon, officials said, so investigators were still working to figure out what information they had before the shooting.

The police union president defended the officers involved in the shooting, saying they were being "vilified" for "doing their job and keeping the community safe."

"The use of lethal force is a very difficult decision and we support our officers as they worked to resolve this incident under a great deal of stress," union President John McNesby said in a statement Tuesday. "These officers were aggressively approached by a man wielding a knife."

But law enforcement experts say the officers' handling of the incident raises important questions.

Whether the shooting was justified or not, "there's a bigger issue at play, there's an issue in how we train police officers to act," said John DeCarlo, an associate professor of criminal justice at the University of New Haven and a former chief of the Branford, Conn., police.

"This is not just a shooting," he said. "And it's not just a right or wrong event here. But it points to a bigger issue of how police interact with communities, how we are training them to interact."

The neighborhood where the shooting happened has had a long and fractious history with law enforcement. A few hundred yards away, the 6200 block of Osage Avenue burst into flames in 1985 after police dropped explosives on houses where members of MOVE, an anti-government black liberation movement, were living. Eleven people, including several children, died. Scores of houses were destroyed.

Monday night's protests began in a West Philadelphia park, not far from where police shot Wallace. Hundreds of people marched through the streets chanting, "Say his name, Walter Wallace," followed by a caravan of honking cars.

As the night wore on, multiple businesses were looted, and a police vehicle was set ablaze.

In the aftermath of the unrest, shattered glass and discarded shoe boxes, pill bottles and clothing hangers littered the street. Los, a 67-year-old handyman from North Philadelphia who declined to give his last name, was working to fix a broken lock at a clothing store Tuesday. He described what happened as "a bunch of kids" with understandable anger.

"There was no need for that boy to die," he said.

Protests that began Tuesday afternoon were led by Black clergy members who gathered to pray on the block where Wallace died and marched, arms locked, to the 18th police precinct.

"Had it been a White man they would have subdued him," said Robert Collier, pastor of Galilee Baptist Church in Roxborough.

About 100 people, many of them White and pushing bikes and strollers, followed the clergy along the streets chanting a call and response of "No justice, no peace" as helicopter blades whirred overhead.

Passing cars honked their horns, and a woman raised her fist from a truck window.

"This is what they should have done last night," she shouted, "instead of tearing up the neighborhood."

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Shepherd, Berman and Witte reported from Washington. The Washington Post's Maura Ewing in Philadelphia and Jennifer Jenkins and Julie Tate in Washington contributed to this report.