CLAYTON, Mo. -- A St. Louis County police officer was shot and killed Thursday, moments after responding to an early-hour disturbance call in a normally quiet, middle-class neighbourhood.

Officer Blake Snyder, 33, was shot point-blank after encountering the man accused of causing the disturbance, Police Chief Jon Belmar said. The 18-year-old man who shot Snyder was inside a car, and a second officer who arrived moments later fired at the suspect, hitting him several times, Belmar said.

The suspect, whose name was not immediately released, is white. He was hospitalized in critical but stable condition. The white officer who shot him was not hurt.

Police were called shortly after 5 a.m. about a disturbance in Green Park, Missouri, in south St. Louis County. Belmar said the 18-year-old was banging on the door of a house demanding to be let in. Police spokesman Shawn McGuire said the suspect had a relationship with a young woman who was inside the house.

Snyder and a second officer were dispatched in separate cars, with Snyder arriving first. Belmar said Snyder saw the suspect in the car and demanded that he show his hands. The suspect then shot Snyder once with a pistol, which was found at the scene, the chief said.

Snyder, a husband with a 2-year-old son, was taken to a hospital and pronounced dead a short time later.

The circumstances that led to the shooting remain under investigation, but Belmar said the suspect was the subject of an earlier drug case in which Snyder was an investigator. Belmar didn't give further details about that case but said of the shooting, "I certainly wouldn't discount the fact that this was an ambush."

Snyder and the other officer were not wearing body cameras and St. Louis County police vehicles do not have dashboard cameras, Belmar said.

Relations between police and residents in St. Louis County have been closely watched since 18-year-old Michael Brown, who was black and unarmed, was fatally shot during an August 2014 confrontation with Darren Wilson, a white Ferguson police officer who was not charged and later resigned.

Green Park, which has about 2,600 residents, is about 24 miles south of Ferguson.

Brown's death led to months of protests that grew violent at times and was a catalyst for the national Black Lives Matter movement and debate over police treatment of minorities. The shooting also prompted a federal investigation into Ferguson's police department and municipal court.

Belmar and other county officers wore black mourning bands over their badges Thursday. People in the halls of police headquarters in Clayton stopped to console each other. A large flag in a courtyard of the facility was lowered to half-mast.

"It's been a tough day," a sombre Belmar said.

Snyder's death "demonstrates the extreme danger that first responders face every day," St. Louis County Executive Steve Stenger said in a statement.

"Our police have my steadfast support and I pledge to do everything I can to provide them with all the resources they need to ensure their safety," Stenger said.

It was the first on-duty death involving a St. Louis County officer since Oct. 31, 2000, when Sgt. Richard Weinhold was killed while investigating a disturbance.