The bloody aftermath of the confrontation was broadcast live on Facebook by a woman driving the car in which Philando was traveling.
"He killed my boyfriend," said the woman, whose Facebook page names her as Lavish Reynolds. In the video, she says Castile was legally licensed to carry a firearm and was reaching for his identification when the officer opened fire.
"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 says in the video.
As a bloody Castile moans and appears to lose consciousness, the officer can be heard in the background shouting expletives in apparent frustration.
"Mam, keep your hands on the wheel," he shouts at Reynolds. "I told him not to reach for it! I told him to get his hands up."
"You told him to get his ID, sir, his driver's license," Reynolds responds. "Oh my god. Please don't tell me he's dead. Please don't tell me my boyfriend just went like that."
By midnight local time, a crowd had gathered at the site of the shooting, according to local television stations.
Castile's family was stunned by the shooting.
"He's gone," Philando's sister, Allysza Castile, 23, said through tears during a brief interview with The Post early Thursday morning.
She said her entire family is gathered at the hospital and as of 1 a.m. had yet to be allowed to see Castile.
"They won't let us see him," she said sobbing. "We've been here probably an hour, the whole family is here, and they won't let us see him."
A spokeswoman for the hospital told the Post early Thursday she had no information on the situation.
Sgt. Jon Mangseth, interim police chief in St. Anthony, a village outside of Minneapolis, said the shooting was the first he could remember in the department's history.
"We haven't had an officer involved shooting in 30 years or more, I'd have to go back in the history books," he said. "It's shocking, it's not something that occurs in this area often."
Mangseth said details of the shooting were unclear.
"As this unfolds we will release the information as we learn it, and we will address concerns as we are made aware of them," he said, adding he had yet to see the Facebook video, which he had only learned about from members of the media.
"As we learn more information we will release that in a press release."