(CNN) Police are on high alert -- and in at least one city, mourning -- after four officers were shot Sunday in incidents around the country.

Perhaps most startling to law enforcement is that in three of the shootings, it appears the alleged perpetrators specifically targeted police officers, according to authorities and local media reports.

An officer in Sanibel, Florida, was shot in a drive-by as he conducted a traffic stop; a Gladstone, Missouri, officer was shot after a traffic stop that produced a foot chase and shootout; a 20-year veteran of the St. Louis Police Department was wounded in what authorities characterize as an ambush; and San Antonio Police Detective Benjamin Marconi was killed outside police headquarters in what appears to be another ambush.

In describing the circumstances leading to Marconi's death, San Antonio Police Chief William McManus cited recent ambush attacks on police in Dallas and Baton Rouge, Louisiana. In the former, sniper Micah Xavier Johnson in July killed five officers protecting activists demonstrating after the police killings of Alton Sterling and Philando Castile. Later that month, Iraq veteran Gavin Long ambushed three officers in Baton Rouge, fatally shooting them.

Marconi's death marked the 60th fatal shooting of an officer in the US this year -- well ahead of the number for all of 2015 (41) but not quite on pace to top the 2011 total of 73, according to the National Law Enforcement Officers Memorial Fund.

Police arrested the suspect in the San Antonio shooting Monday afternoon , while the St. Louis suspect was killed following an exchange of gunfire with police, authorities said. The suspect in Gladstone was killed as well, while the Sanibel suspect was taken into custody after a shootout at a golf and tennis community, local media outlets reported.

Here's what we know about each incident.

San Antonio

JUST WATCHED Texas police look for suspect in cop killing Replay More Videos ... MUST WATCH Texas police look for suspect in cop killing 01:16

Marconi, 50, pulled over a driver outside police headquarters just before noon Sunday and, after addressing the driver, returned to his patrol car to write a ticket.

A man pulled up behind Marconi, exited his vehicle, approached Marconi's car and shot him in the head, McManus said. The suspect then shot Marconi again through the open window of the squad car, the police chief said.

"I believe the uniform was targeted," McManus said Monday at a news conference.

Marconi, who had spent almost half of his life on the force, was killed, and the suspect drove away.

Police arrested 31-year-old Otis Tyrone McKane around 4:30 p.m. Monday while he was riding with a woman and a 2-year-old child in a car on Interstate 10, Chief McManus said.

"This is the person whose image we saw on surveillance," McManus said at a press conference. "This is also the person we believe is responsible for the cold and calculated murder of Detective Marconi."

St. Louis

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Investigators tracked down the suspect in the St. Louis police shooting as he was riding in a friend's car, which officers attempted to pull over, according to police.

The alleged gunman darted from the car, firing once at police, before officers returned fire, killing the suspect early Monday, police said. A pistol with a high-capacity magazine was recovered from the scene.

Authorities did not identify the suspect, but St. Louis Police Chief Sam Dotson said the man was implicated in multiple robberies, a carjacking and a potential homicide. It's likely, Dotson said, that the suspect fired at the police officer Sunday night out of fear of being recognized as a suspect in the string of crimes.

Around 7:30 p.m. Sunday, the gunman pulled up to a marked police SUV and shot a 46-year-old officer twice in the head.

Officer-involved shooting. The suspect is deceased after shooting at officers again. He was wanted for shooting officer earlier tonight. — St. Louis, MO Police (@SLMPD) November 21, 2016

During tonight's investigation, officers pulled behind suspect vehicle when the suspect fled on foot from car & fired shots at officers. — St. Louis, MO Police (@SLMPD) November 21, 2016

Suspect's shots struck officers' vehicle. Officers not shot. Officers returned fire, striking & killing suspect. Suspect's gun recovered. — St. Louis, MO Police (@SLMPD) November 21, 2016

Suspect wanted for violent crimes. We believe suspect shot officer earlier in evening in fear of being recognized. Investigation ongoing. — St. Louis, MO Police (@SLMPD) November 21, 2016

"This officer was not trying to pull this car over," Dotson said. "This officer was driving down the road and was ambushed."

The wounded officer's gun was in his holster during the shooting, and the veteran sergeant and married father of three told police he saw a muzzle flash to his left and felt the glass window shatter against him, Dotson said.

The officer was in critical but stable condition, according to a late Sunday tweet from St. Louis police.

Sanibel

Just before 8 p.m. Sunday, an officer was working a traffic stop when a man drove by and opened fire, according to a police statement.

The suspect then drove to his home in The Dunes on Sanibel Island, a country club community, where he engaged police in a shootout before being taken into custody, police said.

WBBH, Several media outlets, including CNN affiliates WINK WZVN and WFTX , reported that the alleged gunman was wounded, likely in the back and shoulder, during the shootout.

Resident Jeanne Vaughn told WINK she heard six gunshots during the search, which witnesses said included at least 20 police cars and a helicopter.

"I heard a police siren and then, (a) minute later I heard a bunch of shots. Bang, bang, you know, bang, bang, bang," Vaughn told the station. "And then I heard more police sirens and I see police coming up and down the streets."

According to WBBH, the officer, whose name CNN is working to confirm, was lauded as officer of year by the Lee County Bar Association. The officer earned the honor after using a defibrillator to save a woman's life, the station reported.

He has been treated and released from a local hospital.

Gladstone

The Gladstone Police Department officer who was shot late Sunday underwent surgery and is expected to make a full recovery following rehab, the department tweeted.

Because the officer was shot in the Kansas City suburb of Gladstone, police in Kansas City are fielding questions about the case.

The incident began with a traffic violation and routine stop. That is, until a teenage suspect took off running and officers gave chase, said Kansas City Police Department spokeswoman Sgt. Kari Thompson.

Thanks to all for their prayers and calls of support. Our officer is out of surgery and is expected to make a full recovery following rehab. — Gladstone DPS (@gladstonedpsmo) November 21, 2016

Thankfully Gladstone officer shot tonight is expected to live. Not many more details but this is the best news we could have asked for. — KCFOP (@kcfop) November 21, 2016

As officers attempted to take the teen into custody, a struggle ensued.

The suspect, later identified by police as 18-year-old Jacob C. Stevens, brandished a handgun, shots were fired and Stevens was killed, Thompson said. The driver of the vehicle from which Stevens fled was taken into custody, questioned and released.

The injured officer remained at the hospital early Monday, the Gladstone Department of Public Safety tweeted.