A police officer was killed in Texas and another shot in Missouri in apparently unrelated attacks on Sunday, and the suspect in the Missouri incident was killed in a shootout with authorities.

Another police officer was shot and wounded in a similar attack in Florida, although he was not seriously injured.

In Texas, a San Antonio police officer was shot and killed sitting in a squad car during a routine traffic stop outside the city's police headquarters on Sunday, authorities said.

The assailant stopped his car behind the parked police cruiser, walked to the patrol car and shot the officer in the head through the window as he was writing a ticket, Police Chief William McManus said.

The gunman then reached through the window, fired a second shot into the officer, returned to his vehicle and sped away. The slain officer was identified as Benjamin Marconi, 50, a 20-year veteran.

Hours later, a St. Louis policeman was shot in the face as he sat in his cruiser at an intersection, by someone in a car who pulled up beside the officer, opened fire and fled. St. Louis Police Chief Sam Dotson said the wounded officer was conscious and able to speak after the attack.

The St. Louis Post-Dispatch reported the suspect was later killed during a shootout as he fled from officers who spotted his car. He has not been identified.

In Florida, local media reported another police officer was shot during a traffic stop on Sanibel Island, on the state's Gulf coast.

The officer was treated for a shoulder wound and later released, the reports said. The suspect was apprehended at his home on an island off Ft. Myers.

Investigators still lacked any immediate clue to the identity of the San Antonio gunman. They found no apparent link with the man who had been pulled over, McManus told reporters.

"This is everyone's worst nightmare," McManus said. Referring to recent ambush killings of police officers in Texas and Louisiana, he said, "You never want to see anything like this happen. Unfortunately, like Dallas, like Baton Rouge, it's happened here now."

Texas Governor Greg Abbott said in a statement: "Attacks against law enforcement officers will not be tolerated in Texas and must be met with swift justice."

LETHAL FORCE

McManus said police were looking for a man of slim build and dark complexion in his 20s or 30s, possibly with a goatee beard, whose image was captured by security cameras. The gunman made his getaway in a black car with tinted windows.

McManus did not say whether police believe there was a racial element to the shooting. He said San Antonio officers were being ordered to call for backup during traffic stops.

The San Antonio and St. Louis shootings marked the latest in a string of attacks on law enforcement across the country in recent months, at a time of intense public debate over the use of lethal force by police, especially against minorities.

In July, five Dallas police officers were killed when a black U.S. military veteran opened fire in a sniper attack during a protest against police shootings of black men. Days later, a gunman killed three police officers and wounded four others in Louisiana's capital of Baton Rouge.

More recently, an Iowa man who had been ejected by police from a high school football game after waving a confederate flag at black spectators was charged with killing two police officers who were shot in their patrol cars in the Des Moines area.

A total of 57 US law enforcement officers have been killed by gunfire so far this year, a 68 percent increase from the same period in 2015.