A manhunt was underway Monday for the unidentified man who fatally shot a San Antonio police officer at a traffic stop, one of four officers shot across the nation Sunday.

Three police officers were wounded and two suspects killed in shootings in St. Louis and Gladstone, Mo., and Sanibel, Fla.

In San Antonio, Police Chief William McManus said Detective Benjamin Marconi, 50, was conducting a traffic stop outside police headquarters just before noon when he was shot in his car while writing a ticket. McManus said a black male drove up in a Mitsubishi Galant, walked up to Marconi's car and shot the 20-year department veteran in the head before reaching in and shooting him again.

The shooter then drove off, McManus said.

"This is always difficult, especially in this day and age when police officers are being targeted across the country," McManus said. "It's everyone's worst nightmare. Unfortunately, like Dallas, like Baton Rouge, it has happened here now."

In St. Louis, a police sergeant who was shot twice in the face Sunday evening was expected to survive, Chief Sam Dotson said. Dotson said the officer was sitting in his cruiser in traffic when he was shot by someone in a nearby car. The officer was listed in critical but stable condition at Barnes Jewish Hospital in the city.

The 46-year-old sergeant is a husband and a father of three children, Dotson said. The 19-year-old suspect was killed hours later in a shootout with officers. Dotson said the teen may have been connected to several robberies, a carjacking and a homicide.

In Gladstone, an officer was shot after a passenger ran from a car stopped for a traffic violation, according to Kansas City police, which handled the investigation. The suspect, described as a white male in his late teens, pulled out a gun and was killed in the ensuing shootout that left one officer wounded.

"Thankfully Gladstone officer shot tonight is expected to live," the Fraternal Order of Police tweeted. "Not many more details but this is the best news we could have asked for."

In Florida, an officer was in his patrol car wrapping up a routine traffic stop at about 8 p.m. ET Sunday when he was shot by someone driving by. The city said on its website that the suspect was taken into custody shortly after the shooting.

Contributing: KSDK-TV, St. Louis; KENS-TV, San Antonio; The News-Press, Fort Myers, Fla.