McCune, 20, of Euless, Texas, died after allegedly being shot by Vegaquinones after the two men had finished a guard duty shift on Jan. 11 in Samarra, according to a statement.
"The apparent cause of death was a negligent discharge from Vegaquinones' weapon," the military said.
Vegaquinones and McCune were assigned to the 2nd Battalion, 35th Infantry Regiment, 3rd Brigade Combat Team, 25th Infantry Division, which is based at Schofield Barracks, Hawaii.
But Vegaquinones was temporarily moved to the brigade headquarters at the U.S. base near Tikrit pending the outcome of the proceedings. An investigation into the shooting was ongoing, the military said.
Involuntary manslaughter carries a maximum 10-year prison sentence, while lying to investigators can bring a sentence of up to five years, according to the statement.
Samarra, 60 miles (95 kilometers) north of Baghdad, is a former Sunni insurgent stronghold that has seen a drastic drop in violence after local tribal leaders joined forces with the Americans against al-Qaida in Iraq.
It also was the site of a 2006 bombing that destroyed a golden domed Shiite mosque, triggering months of retaliatory sectarian violence nationwide.
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