BERKELEY, Mo. — Violent protests broke out in suburban St. Louis after another black 18-year-old was fatally shot by a white police officer. St. Louis County Police Chief Jon Belmar said the officer was questioning the 18-year-old and another man about a theft late Tuesday at a convenience store in Berkeley when the young man pulled a 9mm handgun on him. The officer stumbled backward but fired three shots, one of which struck the victim, Belmar said. Berkeley is just a few miles from Ferguson, Missouri, where a white police officer fatally shot Michael Brown, an unarmed black 18-year-old, on Aug. 9. Brown's death sparked weeks of sometimes violent demonstrations and a grand jury's decision to not charge Officer Darren Wilson in the shooting has spurred a nationwide movement to protest police brutality. Belmar declined to name the 18-year-old killed in Berkeley, but a woman at the scene told reporters she was his mother and identified him as Antonio Martin. Belmar said he was 18 years old and black. The 34-year-old white police officer, a six-year veteran of the Berkeley Police Department, is on administrative leave pending an investigation, Belmar said. “He will carry the weight of this for the rest of his life, certainly for the rest of his career,” Belmar said. “So there are no winners here.” Police released surveillance video from the parking lot outside the store. The nearly two-minute clip shows two young men leaving the store at about the time a police car rolls up. The officer gets out and speaks with them. About a minute-and-a-half later, the video appears to show one of the men raising his arm, though what he is holding is difficult to see because they were several feet from the camera. Belmar said it was a 9mm handgun. The other man ran away, and police are searching for him. It was the third fatal shooting of a black suspect by a white police officer in the St. Louis area since Brown was killed. Kajaime Powell, 25, was killed Aug. 9 after approaching St. Louis officers with a knife. Vonderrit Myers Jr., 18, was fatally shot Oct. 8 after allegedly shooting at a St. Louis officer. Each shooting has been met by protests, and a crowd quickly gathered late Tuesday and early Wednesday in Berkeley. The demonstration involving up to 300 people turned violent. More than 50 police officers, some in riot gear, responded. Video showed some wrestling with protesters. Belmar said officers used pepper spray but not tear gas. Four people were arrested on charges of assaulting officers. Belmar said three explosive devices, possibly fireworks, were tossed near gas pumps. Some protesters threw rocks and bricks. One officer was hit by a brick and treated for facial cuts. Another was treated for a leg injury sustained as he tried to get away from one of the explosives. The protest spilled to a neighboring convenience store where a man in a hoodie set a fire inside the store. The fire was quickly put out, but the glass door was shattered. Orlando Brown, 36, of nearby St. Charles was among the protesters. “I understand police officers have a job and have an obligation to go home to their families at the end of the night,” he said. “But do you have to treat every situation with lethal force? … It's not a racial issue, or black or white. It's wrong or right.” Brown said he was pepper-sprayed during the protest and that his friend was arrested for failing to disperse. Toni Martin, Antonio Martin's mother, told the St. Louis Post-Dispatch that her son was with his girlfriend at the time of the shooting. The video did not appear to show a female with the two young men. Belmar said the 18-year-old had a considerable criminal record in the less than two years since he turned 17, with three assault charges, armed robbery, armed criminal action and unlawful use of a weapon. The chief said some protesters questioned why the officer couldn't use pepper spray or a stun gun. “Frankly, that's unreasonable,” Belmar said. “When we had somebody pointing a gun at a police officer, there's not a lot of time.” Berkeley has body cameras and dashboard cameras. The officer wasn't wearing his body camera, Belmar said. The dashboard camera activates when the red lights are on, and they were not on at the convenience store. Belmar said the body of the young man remained on the scene for about two hours. After Brown died in August, the fact that his body remained on the street for more than four hours drew widespread criticism. Belmar said two hours is fairly typical as police gather evidence, and he said interference from protesters may have prolonged the situation in Berkeley.