Berkeley is a St. Louis suburb that borders on the city of Ferguson, where the controversial shooting of black teen Michael Brown by a white police officer in August triggered nationwide protests.
Demonstrators appeared after Tuesday's shooting, and there was a scuffle, but the crowd thinned out before daybreak.
KTVI-TV reports the officer feared for his life and fired his weapon in self-defense. Berkeley Mayor Theodore Hoskins , who is black, said Wednesday that this shooting cannot be likened to Brown's death, in part because unlike the Ferguson case, this one was captured on video and allegedly involves an armed teen.
The video below is edited to better show the exchange the officer had with Antonio and the moment Antonio pointed his handgun at the officer.
"We reviewed the video and it appears that there was a gun pointed at the officer before the officer fired," Hoskins said Wednesday. "(It) does not appear ... that the police officer initiated," Hoskins said.
The officer with the City of Berkeley Police Department had encountered two men after responding to a reported theft at a Mobil gas station, Schellman said. He got out of his squad car and approached them.
After engaging the men in conversation, one of the men, identified as Antonio Martin, pointed a gun at the officer. The officer, according to Chief Belmar, lost balance and fell as he moved back in reaction, but managed to fire what investigators believe were three shots, hitting the gunman at least once.
The slain teen is not believed to have fired his weapon, a loaded 9 mm handgun that police recovered at the scene, Belmar said.
The officer took cover behind gas pumps and waited for other officers to arrive before approaching Martin. The second man ran away, police said. They are interested in finding and speaking with him.
The officer is a 34-year-old man and been with the Berkeley Police Department for six years.
The officer was given a body camera before he left for patrol, but he wasn't wearing it during the shooting -- possibly because he was distracted when a supervisor gave it to him earlier Tuesday, Belmar said.
Hoskins said this didn't bother him, in part because the city, with three cameras for its 31 officers, had just started training officers to use them.
The cruiser's dashboard camera also wasn't recording, because the car's emergency lights hadn't been activated, Belmar said. The officer will be placed on administrative leave until the investigation is completed.
Later, yellow markers were placed around the scene at the gas station. Next to one of them was a handgun lying in the parking lot. Schellman said that the killed man's handgun was recovered at the scene.
Feet away from the weapon, a body lay covered up, partitioned off to shield it from public view. It was later placed on a stretcher and loaded into the back of a van.
The gas station appeared to be outfitted with surveillance cameras, which were pointed at the parking lot.
Berkeley police asked St. Louis County police to handle the investigation of the shooting.
Toni Martin told KMOV that her son had gone to see his girlfriend, who lived near the station, and was with her at the time of the shooting. She said the girlfriend notified her of her son's shooting.
Martin said her son turned 18 in September. He had been expelled from school but was trying to get his life back together, she said, and she was encouraging him to join Job Corps.
Video posted to Vimeo showed Toni Martin crying in the arms of other people.
The deceased teen's father, Jerome Green, also said Antonio Martin had said he was going out to meet his girlfriend and had not mentioned any other person who might be with them.
"He was supposed to come home," Green said. "We're getting ready for holiday; everyone wanted to see him. My grandmother hadn't seen him for a while."
Green said that Martin lived with his parents, together with a brother and a sister.
Protesters gathered around an ethnically diverse group of dozens of police officers, who stood between them and the scene. People in the crowd screamed at police. Others spoke more calmly.
St. Louis Post-Dispatch photographer David Carson told CNN's Hala Gorani that some of the protesters damaged police cars. CNN showed images of one squad car with a large dent in its side.
A scuffle broke out, and officers grabbed some of the demonstrators and led them off in handcuffs.
Video posted to YouTube showed two smoking objects flying toward the station, where police stood. One of them emitted white smoke, while the other exploded.
KMOV video showed a white plume rising at another location down the street.
Among the protesters were ministers and people who had participated in Ferguson demonstrations after Brown's shooting death, the Post-Dispatch reported.
Before sunup, protesters thinned out, and those remaining stood calmly behind police tape.