(CNN) - The New York City Police Department released surveillance video Thursday of a man, later fatally shot by police, pointing a silver object at residents as if he were brandishing a gun.
The video also shows 911 call transcripts of neighborhood residents who reported the man to police.
On Wednesday, police shot and killed the black man, identified as Saheed Vassell, in Brooklyn after he pointed what officers believed was a gun at them, authorities said.
After the shooting, officers discovered that the object was "a pipe with some sort of knob on it," Chief of Department Terence A. Monahan said at a news conference.
Vassell's death comes amid a resurgence of questions about law enforcement's unequal treatment of people of color following another police shooting recently in Sacramento, California. Police there said they thought Stephon Clark had a gun, but only a cell phone was found near his body.
New York's attorney general opened an investigation Thursday morning into Vassell's death, said Amy Spitalnick, a spokeswoman for Attorney General Eric Schneiderman.
"We're committed to conducting an independent, comprehensive and fair investigation," she told CNN.
New York City Mayor Bill de Blasio said Thursday the city will be "as transparent as we can in this situation."
He also hypothesized what might have happened if the man actually did have a gun.
"Let's play out the scenario had it been different," he said. "If this individual with a loaded weapon, who for whatever reason, including a mental health challenge, was ready to use it, that's a split-second matter of trying to save lives right then and there.
"How you get the full facts of what the person has in their hand, and what their mental health condition might be, and are they known to anyone, in something that's playing out in seconds and minutes, that's a very tall order," de Blasio said.
'Two-handed shooting stance'
The incident started shortly before 5 p.m. Wednesday when officers received 911 calls of a man aiming what callers described as a silver firearm at people in Brooklyn, Monahan said.
"Three different 911 callers described a man with a gun, pointing it at people on the streets," he said.
When officers arrived at the scene, they found a man matching the description provided by the callers, Monahan said.
"The suspect then took a two-handed shooting stance and pointed an object at the approaching officers, two of whom were in uniform," he said.
Four officers discharged their weapons, striking the man, Monahan said. Then they gave him first aid and called for an ambulance to take him to the hospital, where he was pronounced dead.
"It appears we fired 10 rounds between the four officers," Monahan said. The unidentified officers, who were not wearing body cameras, discovered the metal pipe at the scene.
911 transcripts released
In transcripts released Thursday by New York police, 911 callers appeared somewhat uncertain about what object Vassell was holding.
"There is a guy in a brown jacket walking around pointing -- I don't know, (to someone else) what is he pointing in people's face? They say it's a gun, it's silver," one caller said, according to a police transcript.
"There's a guy walking around the street, he looks like he's crazy, but he's pointing something at people that looks like a gun and he's like popping it, as if, like if he's pulling the trigger," another caller said.
A third caller said the man is holding a gun.
De Blasio emphasized that 911 dispatchers and police were responding to those fear-filled reports.
"If that's what officers were responding to in real time, we've got to recognize that if they believe they are dealing with an immediate matter of life and death to the people in the surrounding area, that's an exceedingly difficult, tense, split-second decision that has to be made," he said.
By state law, the attorney general is appointed as a special prosecutor to oversee investigations into and prosecute matters related to incidents in which unarmed civilians die during interactions with police or incidents in which there is significant question as to whether the civilian was armed and dangerous, Spitalnick said, citing the law.
'He's polite ... kind'
Brooklyn resident Eric Vassell told CNN affiliate NY1 that the victim was his 35-year-old son, Saheed.
Saheed Vassell had no access to guns and suffered from bipolar disorder, his father told the station.
"He's polite, nice, he's kind. He just comes and he goes," his father said.
Mayor Bill de Blasio said his understanding was that Vassell had mental health issues.
"What I understand is the family members have already said publicly this is someone who had a profound mental health problem, was not on medication, hadn't been on medication," he said.
Renewed calls for police reform
Public fury over the shooting deaths of people of color by law enforcement, which gained traction through the Black Lives Matter movement, swelled again last month after police in Sacramento killed Stephon Clark, an unarmed, African-American father.
In light of Clark's killing, which sparked weeks of protests, California lawmakers have proposed a drastic change that would limit the scenarios in which police officers can use deadly force. The bill would replace the "reasonable force" rule with a stricter "necessary force" standard.
The proposal also would establish that a homicide by an officer is "not justified if the officer's gross negligence contributed to making the force 'necessary,'" according to the proposal.
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