By Jim Forsyth

SAN ANTONIO (Reuters) - A police officer was killed in Texas and another wounded in Missouri in apparently unrelated ambush-style shootings, while a third officer was shot and wounded in Florida, authorities said on Monday.

The latest attacks on U.S. law enforcement revived painful memories of deadly ambushes targeting police in July in Dallas and Baton Rouge.

A manhunt was underway for the suspect who killed the officer in San Antonio, Texas, while the suspect in the Missouri shooting died in a shootout with authorities.

In Sunday's first incident, 50-year-old Benjamin Marconi, a 20-year veteran of the San Antonio force was fatally shot as he sat in his squad car during a routine traffic stop outside the city's police headquarters.

The assailant stopped his car behind the police cruiser, walked up 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.

Hours later, a 46-year-old St. Louis police sergeant was shot in the face by someone in a car who pulled up beside the officer's cruiser at an intersection, opened fire, then fled. St. Louis Police Chief Sam Dotson said the wounded officer was conscious and able to speak after the attack.

The suspect was later killed in a shootout after officers spotted his car, police said on Monday.

The unidentified suspect was wanted for other violent crimes and likely shot the officer "in fear of being recognized," police said in a statement.

'WORST NIGHTMARE'

Meanwhile, a third police officer was shot during a traffic stop on Sanibel Island on Florida's Gulf Coast, but was not seriously hurt, local media reported.

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

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Investigators in Texas said they did not have any immediate clues 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 the 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."

McManus said the suspect's image was captured by security cameras.

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 latest shootings come amid an intense national debate over the role of law enforcement and especially the use of force by officers against minorities.

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

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

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

(Additional reporting by Steve Gorman in Los Angeles, and by Chris Michaud and Laila Kearney in New York; Editing by Daniel Wallis and Jeffrey Benkoe)