ST. PAUL, Minn. (CBS NEWS)-- In an emotional statement, a woman who filmed the aftermath of her boyfriend's fatal police shooting during a Minnesota traffic stopsaid her 4-year-old daughter will be forever scarred after witnessing his death.

Philando Castile was killed by an officer Wednesday night after being pulled over for a broken taillight, his girlfriend Diamond Reynolds said Thursday. St. Anthony Police interim police chief Jon Mangseth said the incident began when an officer pulled over a vehicle around 9 p.m. Wednesday in Falcon Heights, a St. Paul suburb that Mangseth's department serves.

Thursday, Reynolds cried as she spoke to a crowd of advocates and reporters outside the governor's mansion, where about 200 protesters had gathered. She said the officer killed Castile as he was reaching into his back pocket to retrieve his identification.

"That was my best friend," Reynolds said. "He killed him in front of us for no reason."

The video posted Wednesday night on Facebook Live shows Reynolds in a car describing the scene next to her bloodied boyfriend quietly slumped in a seat. The officer tells her to keep her hands up and says: "I told him not to reach for it. I told him to get his hand out."

"You shot four bullets into him, sir. He was just getting his license and registration, sir," Reynolds responds.

Reynold's young daughter had been riding in the car. The video goes on to show Reynolds exiting the car and being handcuffed. Her young daughter can be seen and is heard saying at one point, "I'm scared, Mommy."

"I will be okay, but I don't know if I can't say the same for my 4-year-old daughter," Reynolds said Thursday. She demanded justice in the emotional statement, which was also streamed live on Facebook.

She said the girl was not Castile's biological child, but he raised the child as his own. She described the girl as a "warrior" and a "superhero" who was helping her through the emotional aftermath of the shooting.

"She knew he was gone before I did," Reynolds said. "She said 'Mom, the police are bad guys -- they killed him and he's never coming back.'"

Reynolds said they had been driving home after working and then going grocery shopping. Castile had gotten a haircut for his upcoming birthday, which she said was in nine days. Castile was a St. Paul public schools worker who had "never been fingerprinted, never been handcuffed," she said.

Reynolds described being pulled over for broken tail light, which she says wasn't broken. She said the officer asked to see Castile's license, and Castile reached into his back right pocket where he keeps his identification. Castile told the officer he was carrying a firearm, and Reynolds said she told the officer he was legally licensed to carry.

That's when, she said, the officer fired five shots into Castile's chest. She said the officer told them not to move, but asked "how can you not move when they ask you for your license and registration?"

"Nothing within his body language said, 'Kill me, I want to be dead,'" Reynolds said.

She said she believed the officer would have shot her too, had she moved.

Reynolds said she decided to film the aftermath of the shooting so the public could see "what police do" for themselves. She thanked everyone who shared the video, which quickly went viral.

"I didn't do it for pity, I didn't do it for fame," Reynolds said. "I did it so the world would know that these police are not here to protect and serve us. They are here to assassinate us. They are here to kill us because we are black."

After the shooting, Reynolds said, she was taken into custody and separated from her daughter. She said no one told her Castile was dead until 3 a.m., and she wasn't released until 5 a.m. Authorities gave her the wrong information about the hospital Castile was sent to, she said. He was pronounced dead at Hennepin County Medical Center.

Family and friends described Castile as a person who stayed out of trouble.

Castile's grandfather, 89-year-old Sam Castile of St. Paul, said his grandson was a "guy who went about his business," went to work and back home to his mother and sister.

"It's just like we're animals," Castile's sister Alize Castile told CBS News. "It's basically modern day lynching that we're seeing going on except we're not getting hung by a tree anymore - we're getting killed on camera."

In a statement issued Thursday, the governor said the state Bureau of Criminal Apprehension had begun an independent investigation. He said he also spoke with the White House chief of staff to request a separate Justice Department investigation.