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.