Kansas City, Mo. ? A state appeals court on Tuesday overturned the conviction of a man serving an 18-year prison sentence for helping his father kill a driver who had yelled at them when their truck veered into his lane.

In overturning the conviction of Thomas M. Brown Jr., a three-judge panel of the Missouri Court of Appeals’ Western District found the prosecutor had offered what amounted to unsworn testimony during closing arguments when she commented on a defense witness.

Brown, 29, of Kansas City, was convicted in July 2005 of second-degree murder and armed criminal action in the fatal stabbing of Edward A. Chacon, 29, in 2001.

Prosecutors argued that Brown and his father, who each weighed more than 300 pounds, attacked the 115-pound Chacon in a restaurant 15 minutes after they had exchanged words in traffic. Prosecutors said the men spotted Chacon’s car outside a Waffle House restaurant, went inside with a hammer and folding knife, and attacked him.

During Brown’s trial, the defense had argued that he was defending himself and his father, Thomas Brown Sr., who was sentenced to 22 years in prison for his role in the attack.

Waffle House customer Russell Fletcher testified that as the senior Brown attacked, Chacon had tried to stand up and had put his hand on a butter knife for protection but that the knife never left the table.

The defense noted that Fletcher’s testimony differed from a deposition in which he said Chacon had reached down and grabbed the butter knife after standing up. When questioned by the defense, Fletcher denied that assistant prosecutor Amber Gonzalez had coached him to say that the butter knife never left the table.

But Brown’s aunt, Anita Brown, testified that during a break in the elder Brown’s trial she overheard Gonzalez tell Fletcher to testify that the butter knife never left the table.

Over the defense’s objections, Gonzalez discussed the credibility of Anita Brown during closing arguments.

“The idea,” Gonzalez told jurors, “that an officer of the court, a prosecutor, would risk a law license, a career, a law degree, years of had work, on one case is so offensive and completely ridiculous that we ask that you take Mrs. Brown’s testimony for what it is worth, which is nothing.”

The defense argued that the trial court erred when it overturned its objections to Gonzalez’s comments.

The appeals court agreed, writing in its ruling that Gonzalez in effect offered testimony in which she “improperly touted her own credibility to the jury and implied special knowledge to the jury.”

“This unsworn, untested testimony went to a crucial issue in the case – whether she coached a witness to say that Chacon did not pick up the butter knife first. This testimony effectively rebutted Brown’s self defense and defense of a third person defenses because if believed it would prove that Chacon was not the initial aggressor,” the appeals court wrote.

The Jackson County prosecutor’s office would not immediately comment on the ruling.

Brown’s attorney, Kent Denzel, said he was waiting to find out whether the Missouri attorney general’s office planned to appeal to the state Supreme Court. He said that decision would need to be made before prosecutors considered whether to retry the case.