(CNN) As Philando Castile's head slumps backward while he lies dying next to her, Diamond Reynolds looks into the camera and explains a Minnesota police officer just shot her fiancé four times.

The nation is, by now, accustomed to grainy cell phone videos of officer-involved shootings, but this footage from Falcon Heights, outside Minneapolis, is something different, more visceral: a woman live-streaming a shooting's aftermath with the police officer a few feet away, his gun still trained on her bloody fiancé.

"He let the officer know that he had a firearm and he was reaching for his wallet and the officer just shot him in his arm," Reynolds said as she broadcast the details of Wednesday's evening shooting on Facebook.

Castile, an African-American, was a school nutrition services supervisor who was popular among his colleagues and students, according to his employer.

He had been pulled over for a broken taillight, Reynolds explained on the Facebook video. He told the officer he was armed and had a concealed carry permit, she said. Her daughter, 4, was in the back seat.

As she speaks, Castile's wrists are crossed. Blood covers the bottom of his white T-shirt sleeve and a large area around his sternum and left rib cage. Perhaps in shock or agony, he peers emptily upward. At one point, he moans in pain as she describes the situation.

'You shot four bullets into him, sir'

Though you can't see the St. Anthony police officer's face, you can hear the agitation in his voice as he tells Reynolds to keep her hands visible.

Composed, as she remains through much of the video, Reynolds replies, "I will, sir, no worries. I will."

The officer still sounds distressed as he explains, "I told him not to reach for it. I told him to get his hand off it."

Moments later, Reynolds pleads with God and then the officer as she realizes Castile won't make it.

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"Please don't tell me this, Lord. Please, Jesus, don't tell me that he's gone," she said. "Please, officer, don't tell me that you just did this to him. You shot four bullets into him, sir. He was just getting his license and registration, sir."

She continues pleading outside the car as officers approach her with guns drawn. One orders her to her knees. The phone films the sky.

"Please Jesus, no. Please no. Please no, don't let him be gone," Reynolds says before officers place her and her daughter in a police cruiser.

Later, at Hennepin County Medical Center, her fears were confirmed: Her fiancé was gone, just a week and a half before his 33rd birthday.

President Barack Obama, speaking in Warsaw at a NATO summit, called the shooting deaths of Sterling and Castile "tragedies" and demanded that the country as a whole "do better." Americans should feel outraged at episodes of police brutality since they're rooted in long-simmering racial discord, Obama said.

A mother's anguish

Castile's mother said that he and his sister had stopped by her house earlier Wednesday. During the visit, they had discussed the dangers of carrying weapons, even though both of them have concealed carry permits.

"I really don't even want to carry my gun because I'm afraid that they'll shoot me first and then ask questions later," Valerie Castile eerily recalled her daughter saying.

I know he didn't antagonize that officer in any way to make him feel like his life was threatened. Valerie Castile, mother of Philando Castile

She learned of the shooting via phone calls from people witnessing the live stream on Facebook, she said. When she and her daughter arrived on the scene, they weren't permitted to speak to Reynolds, she said.

By the time she arrived at the hospital, the grieving mother said, her son was already dead and authorities wouldn't let her see him or identify him. Police won't let her ID him until Friday, she said.

"I'm not getting the answers that I'm asking for," she said. "They're telling me that they don't know anything, so I don't know anything."

A community vigil and march was held Thursday evening, beginning at J.J. Hill Montessori Magnet School in St. Paul.

Uncle: Police are supposed to protect us

Philando Castile's uncle, Clarence Castile, said the images of his nephew dying are the "most horrific thing I've ever seen in my life." He, too, cast a critical eye on the nation's police.

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"We hear about things like this happening all the time around the United States and the world, people being harmed and abused by people that we're supposed to trust with our lives, people that are supposed to serve and protect us. And they tend to be our executioners and judges and murderers."

Clarence Castile said his nephew was "so docile and laid back," it's difficult to see how anyone might perceive him as threatening.

An ongoing investigation

Sgt. Jon Mangseth, interim chief of the St. Anthony police, said two officers were present when the shooting occurred. They were identified late Thursday as Officer Jeronimo Yanez and Officer Joseph Kauser.

Both have been with the St. Anthony Police Department for four years. Both are on standard administrative leave.

"Officer Yanez approached the vehicle from the driver's side and Officer Kauser from the passenger side. At one point during the interaction, Officer Yanez discharged his weapon, striking Castile multiple times," according to the Minnesota Department of Public Safety Bureau of Criminal Apprehension.

St. Anthony police don't have body cameras, according to office manager Kim Brazil.

One officer has been placed on paid administrative leave, which is also standard procedure, Mangseth said at a news conference. No police were injured.

"We will release the information as we learn it, and we will address concerns as we are faced with them," he said.

Mangseth said early Thursday he hadn't seen the video, but he knew about it. The nearly 10-minute video garnered more than 1 million views before it was pulled from Facebook. It was then re-released on the social media platform with a graphic warning.

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The Minnesota Bureau of Criminal Apprehension Assistance is investigating, Mangseth said. An autopsy was under way Thursday at the Hennepin County Medical Examiner's Office, a spokeswoman said.

Protests, vigil

By early Thursday, protesters had begun gathering outside Minnesota Gov. Mark Dayton's residence.

Minneapolis amateur photographer Tony Webster shot photos late Wednesday of gatherings at the scene of the shooting and at the governor's mansion. He described two very different atmospheres.

In Falcon Heights, word of Castile's death hadn't reached the protesters, and while the scene was "incredibly intense," those congregating were mostly observing and recording the investigation unfolding behind the police tape.

"It was very sober. There were candles. There were signs, but they weren't politically charged," he said.

At the governor's mansion, he said, people demanded accountability. Webster felt compelled to document it because he hoped the photos would "bring people to the conversation," he said.

"I live in Minneapolis, so I think these issues hit home," he said. "I'm white and I don't have these experiences myself, but it's tough. No one who is white will ever understand the feeling of being pulled over and fearing for their life."

Photos: Reaction to recent police shootings Photos: Reaction to recent police shootings Protesters sit after being arrested after a march on July 10 in Baton Rouge, Louisiana. Many cities have seen an increase in protests since police-involved shootings. Hide Caption 1 of 15 Photos: Reaction to recent police shootings Demonstrators gather on July 9 at the Louisiana Capitol to protest the shooting of Alton Sterling in Baton Rouge. Hide Caption 2 of 15 Photos: Reaction to recent police shootings A couple hold a sign protesting the killing of Philando Castile outside the governor's mansion on July 7, in St. Paul, Minnesota. Castile's death was live-streamed by his fiancée and quickly went viral. Hide Caption 3 of 15 Photos: Reaction to recent police shootings Protesters gather Wednesday, July 6, in front of the convenience store where Alton Sterling was fatally shot in Baton Rouge. Sterling, 37, was shot by police on Tuesday, July 5, and federal authorities are investigating what happened. Vigils and memorials spread across the country after cell phone video of the shooting was shared widely on social media. Hide Caption 4 of 15 Photos: Reaction to recent police shootings One of Sterling's children, Cameron, is comforted by a crowd outside the convenience store on July 6. Hide Caption 5 of 15 Photos: Reaction to recent police shootings Tawandra Carr, who said she was best friends with Alton Sterling, cries while people gather outside the convenience store on July 6. Hide Caption 6 of 15 Photos: Reaction to recent police shootings A woman joins others gathered at the scene of a police-involved shooting in Falcon Heights, Minnesota, on July 6. Philando Castile, 32, was fatally shot by police during a traffic stop. His fiancee, Diamond Reynolds, live-streamed the aftermath on Facebook. The shooting is being investigated. Hide Caption 7 of 15 Photos: Reaction to recent police shootings A crowd of community members gathers outside the governor's residence in St. Paul, Minnesota, following Castile's shooting. Hide Caption 8 of 15 Photos: Reaction to recent police shootings Community members and activists covered the governor's residence with police tape from the scene where Castile was killed. Hide Caption 9 of 15 Photos: Reaction to recent police shootings People in Baton Rouge demonstrate July 6 as police cars pass the convenience store where Alton Sterling was shot. Hide Caption 10 of 15 Photos: Reaction to recent police shootings Jordan Nuerenberg holds up a sign encouraging motorists to honk their horns outside the Baton Rouge convenience store. Hide Caption 11 of 15 Photos: Reaction to recent police shootings Protesters block traffic and dance on cars near the convenience store. Hide Caption 12 of 15 Photos: Reaction to recent police shootings A woman rings the doorbell at the gate of the governor's home in Minnesota after Castile's shooting on July 6. Hide Caption 13 of 15 Photos: Reaction to recent police shootings Community members and activists gather outside the Minnesota governor's home. Hide Caption 14 of 15 Photos: Reaction to recent police shootings Someone uses their phone to live-stream the investigation taking place at the scene of Castile's shooting. Hide Caption 15 of 15

Dayton released a statement saying he extended his condolences to those who knew Castile and later, during a news conference, questioned the officer's response.

"Nobody should be shot and killed in Minnesota for ... a taillight being out of function. Nobody should be shot and killed while they are seated still in their car without a very, very different kind of response," he said.

Dayton said he didn't think the shooting would have occurred if Castile and others in his car had been white.

The U.S. Justice Department released a statement saying it "is aware of the incident and is assessing the situation."

'I'm right here'

As Reynolds narrates the shooting on Facebook, she's calm and composed at first -- a juxtaposition to the officer outside the vehicle.

Outside Castile's car, Reynolds begins to cry and lose composure. She wails. Police can be heard in the background. The camera keeps pointing up at the sky, before it goes black while the voices continue.

Reynolds eventually begins filming from the back seat of the police car with her little girl. She seems calm again, alerting viewers to her location and asking that someone pick her up.

"I can't believe they just did this," she says.

Then she screams, her anguish clear.

"It's OK," the little girl says. "I'm right here with you."

Reynolds credited her 4-year-old with the calm demeanor she displayed on Facebook after the shooting.

"My daughter was my lifeline. My daughter told me to stay strong. My daughter told me, 'Don't cry,' and that's what I had to do," she told reporters. "Without this little angel by my side, I would never have been able to make it through this."

Castile was not the girl's father, "but he raised her and took care of her as if she was his," she said.

Reynolds asked that people pray for her daughter "because I will be OK, but I don't know if I can say the same for my 4-year-old daughter."

'I wanted it to go viral'

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In fiery remarks to reporters during an impromptu Thursday news conference outside the governor's residence, Reynolds lambasted the St. Anthony police, saying they separated her from her daughter, didn't tell her until 3 a.m. that Castile was dead and didn't take her home until 5 a.m.

"They took me to jail. They didn't feed us. They didn't give us water," she said. "They put me in a room and separated me from my child. They treated me like a prisoner. They treated me like I did this to me and I didn't. They did this to us."

She and Castile had just left the grocery store when they were pulled over. She cast doubt on the alleged reason for the traffic stop.

"The police officer stopped us for a busted taillight that wasn't busted," Reynolds said. "They asked him for identification and before they gave him a chance to get it, they bared arms on him. He took his last breath in front of us, where he died on the scene."

She said Castile was reaching into his back pocket for his identification when the officer opened fire.

"They took an innocent man from us. He didn't do anything," she said. "He did exactly as the police asked."

She said the officer should "not be home with his family" and she would like to see him jailed. She further expressed disgust that while officers placed her in the back of the police cruiser, other police were consoling the officer who shot Castile, telling him he'd be OK.

Asked why she began live-streaming after the shooting, she said she wanted people to know the truth.

"I wanted it to go viral so the people could see," she said. "I wanted everybody in the world to see what the police do."