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Four policemen have been shot in 24 hours across the US in the latest in a series of attacks on law enforcement.

One officer was shot dead in San Antonio, Texas, and another was shot in the face in St Louis, Missouri, though he is expected to survive.

At least two of Sunday's attacks were ambushes. Two other officers were shot elsewhere in Missouri and Florida.

The attacks revived memories of deadly ambushes against police in Texas and Louisiana in July.

Fifty-seven US law enforcement officers have been fatally shot this year, a 68% percent increase from the same period in 2015.

San Antonio

Detective Benjamin Marconi, 50, had been with the force for two decades. He was writing out a traffic ticket when he was fatally shot in his car on Sunday morning.

The brazen attack, by another driver who pulled up from behind, happened outside police headquarters, authorities said.

Police are searching for a male suspect, whose motive was unclear.

San Antonio Police Chief William McManus said he does not suspect the gunman knew the original motorist who was pulled over.

Texas Governor Greg Abbott called the killing a "horrific act of violence".

Missouri

A St Louis police sergeant is in a critical condition after he was shot twice as he sat in traffic in a marked police vehicle at about 19:30 on Sunday.

"This officer was driving down the road and was ambushed by an individual who pointed a gun at him from inside of his car and shot out the police officer's window," Police Chief Sam Dotson told a news conference.

The unidentified officer is a 46-year-old, married father-of-three who has been with the department for about 20 years.

The suspect was killed in a shootout with police.

A Gladstone, Missouri, police department officer was also shot near Kansas City.

Authorities said the police officer suffered non-life threatening injuries during an exchange of fire with a suspect who fled during a traffic stop.

The suspect was fatally shot.

Florida

In Sanibel, Florida, one policeman was targeted by a drive-by attacker at around 20:00 on Sunday as he sat in his patrol car after a routine traffic stop.

The officer was treated for his injuries and released; a suspect was later shot and taken into custody.

The attacks came less than five months after a gunman killed five officers in Dallas, Texas, in the deadliest day for US law enforcement since the 9/11 attacks.

A few days after the Dallas attack, three police officers were ambushed and killed in Baton Rouge, Louisiana.

Earlier this month, two officers in the Des Moines, Iowa, area were shot dead in ambushes as they sat in their patrol cars.