A state crime lab chemist returned to the witness stand for a second day Tuesday as a trial resumed for a man who served time for a deadly 1998 school yard ambush and now faces federal charges that could return him to prison for a decade. Senior chemist Gary Dallas acknowledged in testimony Monday that an initial exam of a material found in Mitchell Johnson's pocket came back negative for marijuana, but a lab worker mistakenly recorded the results as positive. Johnson, 23, faces one count of possessing of a firearm while either being a user or being addicted to a controlled substance. It carries a maximum penalty of 10 years in prison.

Dallas testified Monday that he conducted a second test of the drugs last week before Johnson's federal trial on a weapons charge began Monday, and that test was positive for marijuana. Dallas said he conducted the test after workers realized the first report contained a "clerical error."

Johnson was arrested on New Year's Day 2007 after police stopped his van and 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 that there were drugs inside.

Assistant U.S. Attorney Clay Fowlkes said the marijuana and the weapons found during the traffic stop made him guilty. Fowlkes said other witnesses would testify they saw Johnson constantly high at home and at work.

John B. Schisler, Johnson's lawyer, said deputies only looked for a reason to pull Johnson's van over after receiving a fraudulent tip that it carried 100 pounds of marijuana. Instead, Schisler said Johnson and friend Justin Trammell were moving to California, "a journey that got cut short by a lie."

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 students Natalie Brooks, 11; Paige Herring, 12; Stephanie Johnson, 12; and Britthney Varner, 11. They wounded 10 other people.

State courts sent Johnson and Golden to a juvenile prison until their 18th birthdays. But federal prosecutors swept in before their birthdays, locking them up until they turned 21.