The latest attacks on US law enforcement revived painful memories of deadly ambushes targeting police in July in Dallas and Baton Rouge.
A fourth officer in Kansas City, Missouri, was shot in a struggle with an armed suspect, but he may have been shot by a fellow officer.
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.
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, according to the reports, while the suspect was apprehended at his home on an island off Ft. Myers.
'Worst nightmare'
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.
In the fourth incident, a police officer in Kansas City was shot and wounded during a struggle with an armed suspect who tried to flee after a traffic stop, police said.
The suspect was killed and a Kansas City police spokeswoman said investigators were determining whether the officer was shot accidentally by one his colleagues.
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.
A person of interest was being questioned in connection with the killing, the US Marshals Service said on Monday, but police said the suspect was still being sought and no one had been arrested.
"This is everyone's worst nightmare," McManus said. Referring to the recent ambush killings of police officers in Texas and Louisiana, he said, "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 US 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.
