A white former St. Louis police officer was found not guilty of first-degree murder in the death of a black man who was shot to death during a 2011 high-speed chase, a verdict which stirred feelings of anger in the local community.

Missouri Gov. Eric Greitens placed the National Guard on standby Friday after a judge acquitted former officer Jason Stockley. Local and state officials braced for violent protests similar to those that followed the 2014 fatal shooting by police of unarmed black teenager Michael Brown in Ferguson.

Stockley, 36, could have been sentenced to up to life in prison without parole for shooting 24-year-old Anthony Lamar Smith five times after a high-speed chase. He told the court he saw Smith holding a gun and felt he was in imminent danger.

Prosecutors, however, accused Stockley of planting the gun in Smith’s car after he was shot.

Stockley, 36, could have been sentenced to up to life in prison without parole. He left St. Louis' police force in 2013 and moved to Houston.

Assistant Circuit Attorney Robert Steele emphasized during the trial that police dashcam video of the chase captured Stockley saying he was "going to kill this [expletive], don't you know it." Less than a minute later, the officer fatally shot Smith. Stockley's lawyer dismissed the comment as "human emotions" amid a dangerous police pursuit.

Few officers have been convicted for killing suspects while on duty. Stockley requested the case be heard by a judge rather than a jury despite objections from prosecutors.

Circuit Judge Timothy Wilson, who oversaw the bench trial, said prosecutors also asked the court to consider a lesser degree of homicide. But Wilson said prosecutors did not prove beyond a reasonable doubt the officer’s use of deadly force was not justified in self-defense.

“This court, as a trier of fact, is simply not firmly convinced of defendant’s guilt,” the judge wrote in his ruling.

Ahead of Friday’s verdict, St. Louis activists threatened civil disobedience if Stockley was acquitted. The mayor and an attorney for Smith’s fiancé publicly urged for calm.

Barricades went up on Aug. 28 around police headquarters, the courthouse where the trial was held, and other sites of recent or potential protests. Police said they were being proactive to ensure safety "due to recent events around the country."

Michael Brown’s father, Michael Brown Sr., voiced his frustration after the verdict.

“You all know this ain’t right and you all continue to do this to us,” he told FOX2. “Like we don’t mean nothing, like we’re rats, trash, dogs in the streets. Right now, I‘m praying for my city because my people are tired of this.”

The police officer in the Brown case wasn’t charged but later resigned.

“I’m sad, I’m hurt, I’m mad,” the Reverend Clinton Stancil of the Wayman AME Church in St. Louis told Reuters. “But this was expected. We haven’t made any progress since Ferguson, that’s clear. Cops can still kill us with impunity.”

In Smith's case, the encounter began when Stockley and his partner tried to corner Smith in a fast-food restaurant parking lot after seeing what appeared to be a drug deal. Stockley testified he saw what he believed was a gun, and his partner yelled "gun!" as Smith backed into the police SUV twice to get away.

Stockley's attorney, Neil Bruntrager, argued Smith, a parole violator with previous convictions for gun and drug crimes, tried to run over the two officers. Stockley fired seven shots as Smith sped away. A chase ensued.

At the end of the chase, Stockley opened fire only when Smith, still in his car, refused commands to put up his hands and reached along the seat "in the area where the gun was," Bruntrager said. Stockley said he climbed into Smith's car and found a revolver stuffed between the center console and passenger seat.

But prosecutors questioned why Stockley dug into a bag in the back seat of the police SUV before returning to Smith's car.

The gun found in Smith's car didn't have his DNA on it, but it did have Stockley's.

"The gun was a plant," Steele said.

The case was among several in recent years in which a white officer killed a black suspect. Officers were acquitted in recent police shooting trials in Minnesota, Oklahoma and Wisconsin. A case in Ohio twice ended with hung juries, and prosecutors have decided not to seek a third trial.

The Associated Press contributed to this report.