A suburban St. Louis police officer was in critical condition late Friday after he was shot in the neck in what investigators described as an "ambush" following a traffic stop.

Ballwin Police Chief Kevin Scott said the unidentified officer was walking to his car after the initial conversation with the motorist he stopped for speeding Friday morning when that driver "advanced quickly" on him from behind, firing at least three shots.

The officer "had no chance at all" to pull his handgun and "was completely helpless," Scott said, noting the encounter was recorded by the police car's dashcam.

"Make no mistake: We believe during this investigation that Ballwin officer was ambushed, period," St. Louis County Police Chief Jon Belmar said.

Scott said the officer was in critical but stable condition, and "fighting for his life" at a hospital in Creve Coeur, west of St. Louis.

After the shooting, Belmar said, the suspect sped away before an officer from another police department spotted the car about 4 miles away. The suspect abandoned his vehicle and fled on foot before being arrested about five minutes later, Belmar said.

Antonio Taylor, a 31-year-old black man who was paroled in 2015 after serving time on a federal weapons charge, was charged with assault of a police officer, armed criminal action and a felon in possession of a firearm, St. Louis County Prosecutor Bob McCulloch said Friday.

McCulloch said there's no evidence of any dispute between Taylor and the white officer, a nine-year veteran before the gunfire. Scott said he "can't even begin to speculate" about a motive, including whether the shooting had racial overtones.

The shooting followed the previous night's attack in Dallas that killed five officers and wounded seven during a protest over the deaths of black men killed by police this week in Louisiana and Minnesota.

Citing his concern about the shooting and the Dallas tragedy, Missouri Gov. Jay Nixon opted against leaving Friday for an eight-day overseas trade mission as planned and instead would return to Missouri from a Philadelphia event, spokeswoman Channing Grate said.

The shootings of officers in Ballwin, Dallas, Tennessee and Georgia in a 24-hour period prompted police agencies regionally and elsewhere in the U.S. to take precautionary safety measures. Earlier Friday, St. Louis Police Chief Sam Dotson said his city's law officers will work in pairs until further notice because of the Dallas killings, and that all officers must wear bullet-resistant vests when on duty outside of police stations.

Belmar said his department has gone to 12-hour days now through the weekend, given the national debate about policing and minorities.

"It's an unfortunate state of events we're dealing with right now," he said. "I do understand the silent majority out there supports us."

Taylor is being held on $500,000 cash bond and is expected to be arraigned on the felony charges Monday morning.

The Associated Press contributed to this report.