"Expert" is the highest level of achievement in marksmanship in the Army.
The standard test for expert marksman involves "knocking down" a minimum of 36 targets with only 40 rounds of ammunition, from distances of 50 to 300 meters, the official said.
How often and in what years he qualified, as an expert is unclear from his record, according to the official.
Williams, who adopted the new last name "Muhammad," was also qualified expert in throwing hand grenades.
Williams and his stepson John Lee Malvo were detained early Thursday in connection with the sniper shootings in the Washington metropolitan area, which have claimed 10 lives and gravely injured three other people since early October.
Williams, a veteran of the Persian Gulf War, served on active duty in the Army from Nov. 6, 1985 to April 26, 1994, when he was discharged at Ft. Lewis, Washington.
He achieved the rank of sergeant.
The circumstances and condition of his discharge are not publicly releasable, according to the defense official.
He also served in the Louisiana National Guard from 1978 to 1985, and in the Oregon guard from 1994 to 1995.
Throughout his active duty career, he was a combat engineer, a broad field encompassing mine laying, removal, demolition and combat construction. He was also a metal worker and a water transport specialist.
In 1985 he was assigned to the 15th Engineer Battalion at Ft. Lewis. In 1990, he was assigned to the 84th Engineering Company in the 2nd Armored Cavalry Regiment, Nuremburg, Germany.
In 1992 he was reassigned to the 13th Engineering Battalion at Ft. Ord, Ca. In 1993 he returned to Ft. Lewis as a member of the 14th Engineering Battalion.
He was awarded the Southwest Asia Service Medal and the Kuwait Liberation Medal for his participation in the Persian Gulf War, the Army Achievement Medal; the Army Service Ribbon, the National Defense Service Medal, and the Non-Commission Officer Professional Development Ribbon.
Williams went through basic and advanced engineering training in 1979, was trained in basic winter operations in 1987, and completed a primary leadership development course in 1988.