Updated

A federal jury convicted a 23-year-old man on an obscure weapons charge Tuesday, 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 Tuesday.

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.

U.S. Attorney Bob Balfe told reporters after the verdict that lawyers picked the jury carefully. He said he was hopeful that if any jurors realized who Johnson was, they could separate the 1998 killings from last year's bust.

"We strongly believe that Mitchell Johnson is a person who should not have a gun," Balfe said. "Particularly, (he) shouldn't have a gun while using controlled substances."

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. Some jurors said they had heard the name but didn't know why.

Mitch Wright, the widower of the teacher killed March 24, 1998, watched Tuesday'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," 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 Tuesday 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.

A prosecution witness testified Tuesday that Johnson often smoked marijuana, but he later acknowledged that he lied under oath when he denied that he personally owned a gun.

Dustin David Wynboom said that he and Johnson smoked marijuana once every week or two and that they once smoked marijuana while working at a local Wal-Mart. Wynboom, 21, of Springdale, said he also saw Johnson's handgun and said Johnson told him "he needed it for protection, that people were after him."

Wynboom denied ever owning a handgun himself when questioned by Johnson's lawyer, John B. Schisler, prompting a long recess. When court resumed, Wynboom admitted he lied under oath.

"I was scared I was going to incriminate myself," Wynboom said.

Another Wal-Mart co-worker, Michael Lindsey, testified Johnson often discussed smoking marijuana and sometimes appeared to be high at work. Lindsey later admitted getting so drunk at a party with Johnson and other Wal-Mart employees that he fell down the stairs.

A senior chemist with the state Crime Laboratory took the stand again Tuesday and acknowledged that an initial exam of material found in Johnson's pocket came back negative for marijuana, although a lab worker mistakenly recorded the results as positive. Chemist Gary Dallas said he tested the material again last week, confirming that it was 21 grams of marijuana.