WATCH: Girlfriend of Minnesota man killed by police during traffic stop gives impromptu speech

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(CNN) -- Diamond Reynolds was fiery and emotional as she talked for the first time about the death of her boyfriend.

Reynolds reignited the debate on fatal police shootings of black men when she live-streamed Facebook video of the aftermath of Philando Castile's killing.

In a dramatic, impromptu news conference Thursday morning, she accused the St. Anthony, Minnesota, police of murdering Castile right before her eyes.

"They took his life for no reason," Reynolds told a small crowd of supporters, media, and onlookers who had gathered around her on the grounds of the Minnesota Governor's Residence in St. Paul. "He took his last breath in front of us."

The traffic stop

Police pulled Castile and Reynolds over in a traffic stop Wednesday evening in Falcon Heights, Minnesota, near Minneapolis.

Reynolds said she and Castile were returning from the grocery store when police stopped them.

"The police officer stopped us for a busted taillight that wasn't busted," she said, adding they were calmly following the officer's instructions. "We had our hands in the air."

Castile told the officer he had a permit to carry a weapon, according to Reynolds' account. The officer asked for Castile's license and registration and "as he was reaching for his ID in his back pocket" shot him multiple times, she said.

She then pulled out her phone and started the live stream video, viewed more than 3 million times on Facebook.

"I wanted it to go viral so the people could see" what happened, she said, explaining why she did that during such a stressful time. "I wanted everybody in the world to see what the police do."

In the video, Reynolds speaks and turns her camera to Castile, who's wearing a blood-covered white shirt. A police officer is visible out the window of the car, with his gun drawn. The footage was streamed on her Facebook account under the name Lavish Reynolds.

'The treated me like a prisoner'

On Thursday, Reynolds accused the St. Anthony police of racism and poor treatment of her and her young daughter, who was also in the car at the time of the shooting.

"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."

Reynolds said police dropped her off at her home at 5 a.m. Thursday.

The officer who she said shot her fiancé cried after the shooting. She said she wants him arrested and charged with murder.

"(He) should not be home with his family. He should be in jail in handcuffs."

Castile was a good man, she said, who worked for a local school district and had never been in trouble with the law.

She said she still can't believe what happened to him.

"He didn't do anything; he did exactly as the police asked," Reynolds cried. "I want justice."