Officers with the San Francisco Police Department have their "eyes on a swivel," a spokesperson says, braced for action following a day in which at least four cops were shot in states other than California.

According the the Ex, SFPD is on "high alert" following four incidents across the US on Sunday in which police officers were shot, one fatally.

On Sunday in San Antonio, Texas, Detective Ben Marconi "was shot at close he wrote a traffic ticket outside of police headquarters," CNN reports. The alleged shooter, 31-year-old Otis Tyrone McKane "killed [Marconi ] specifically because he was a law enforcement officer."

Also on Sunday, 19-year-old George P. Bush III allegedly "pulled up in a silver car next to a marked Chevy Tahoe and opened fire" in St. Louis, CNN reports, striking a "46-year-old sergeant and 20-year veteran of the force...twice in the head." That officer, who has not been publicly named, is expected to survive. He was "ambushed," St. Louis Police Department Police Chief Sam Dotson says, by a suspect who "likely shot the officer Sunday night out of fear he would be recognized for his role in the recent string of crimes."

A police officer in the small town of Santibel, Florida, was also shot Sunday, the city's first reports the News-Press. In that case, the officer "was wounded by a drive-by shooter as he sat in his patrol car after completing a routine traffic stop near Bailey and Sandcastle roads." 49-year-old Jon Webster Hay has been arrested in the case, and charged with attempted homicide.

Finally, 18-year-old Jacob Stevens was shot and killed by police in Gladstone, Missouri, after he allegedly opened fire on two officers after the pulled over the car in which he was a passenger. Speaking with KMBC, Kansas City Police Officer Darin Snapp says that incidents like Sunday's are "our absolute worst nightmare...We’re going to have to watch everybody that walks by us, every car that drives by, because you just never know."

Those incidents have not going unnoticed in San Francisco, where SFPD spokesperson Officer Carlos Manfredi tells the Ex that “These incidents happened when the officers were caught off guard, just going about their business...They were basically attacked from surprise by behind.”

According to Manfredi, even though officers with the SFPD almost always patrol in pairs, "the department will ensure officers are not riding alone in the immediate aftermath of the shootings," reports the Ex. "The public is also asked to remain on the lookout for anything suspicious, and to make note of license plates or suspect descriptions if a crime is witnessed," the Ex reports.

And in the end, Manfredi says, it all comes down to staying frosty. Attacks on officers (for example, the shooting of SFPD Officer Kevin Downs near Stern Grove last month) are, Manfredi says, "one of those things where it’s really hard to predict...You’ve got to keep your eyes on a swivel and look for anything out of the ordinary."

Related: Man Who Allegedly Shot SFPD Officer Had Been Living In A Van Parked In The Sunset