FAYETTEVILLE, Ark. — A federal jury convicted a 23-year-old man on an obscure weapons charge yesterday, apparently unaware that 10 years ago he and another boy killed four classmates and a teacher in a schoolyard ambush. Mitchell Johnson faces 10 years in prison and a $250,000 fine when he is sentenced in the next 45 days on a count of possessing a firearm while being a drug user. Through his attorney, Johnson declined to comment yesterday. Johnson was arrested on New Year’s Day 2007 after police stopped his van and said they found a bag of marijuana in his pocket and a 9-mm pistol and a 20-gauge shotgun in two bags. Police said they stopped the van after getting an anonymous tip about drugs in the vehicle.

In 1998, Johnson, then 13, and 11-year-old schoolmate Andrew Golden opened fire as students and teachers left Jonesboro Westside Middle School after Golden pulled the fire alarm. The boys killed English teacher Shannon Wright and four students ages 11 and 12. They wounded 10 other people. Government lawyers did not bring up Johnson’s violent past. The only clue during the two-day trial came during jury selection, when potential panelists were asked whether Johnson’s name sounded familiar.

Mitch Wright, the widower of the teacher killed March 25, 1998, watched yesterday’s court session along with his son Zane, who was 2 at the time of the shootings. The boy “wanted to see what this person looked like,” Ms. Wright said during a break in the trial.

State courts sent Johnson and Golden to a juvenile prison until their 18th birthdays. Federal prosecutors then got them locked up until they turned 21.

Johnson left prison with an “adjudicated” record — meaning he could own firearms.

Prosecutors in the trial that ended yesterday presented evidence that Johnson regularly used marijuana. The defense offered drug tests that showed no drug usage by Johnson and testimony from witnesses who said they had never seen Johnson use drugs or possess drug paraphernalia.