BAGHDAD, Iraq - A U.S. Army staff sergeant was charged with murdering his two commanders last week at a base outside Baghdad, the military said Thursday in what is believed to be the first case of an American soldier in Iraq accused of killing his superiors.

The military initially concluded that the June 7 deaths of Capt. Phillip T. Esposito, of Suffern, N.Y., and 1st Lt. Louis E. Allen, of Milford, Pa., were caused by a mortar round.

But on Wednesday, the military charged Staff Sgt. Alberto B. Martinez of Troy, N.Y., with two counts of premeditated murder, according to a statement issued in Baghdad.

Martinez, 37, is a supply specialist with the Headquarters Company of the 42nd Infantry Division, New York Army National Guard. Esposito, 30 and the father of a 1-year-old girl, was company commander and Allen, 34 and a father of four, was a company operations officer.

The "fragging" incident occurred near Tikrit - Saddam Hussein's hometown 80 miles north of Baghdad - at Forward Operating Base Danger in what used to be one of the ousted Iraqi leader's palaces on the banks of the Tigris River.

Fragging is a term used to refer to soldiers killing their superiors.

A criminal investigation was launched after it was determined that the "blast pattern" at the scene was inconsistent with a mortar attack.

Martinez is believed to have allegedly used some kind of explosive device, possibly a grenade, in the attack, military officials said on condition of anonymity because the matter was still under investigation.

He was charged with two counts of premeditated murder, said a statement by the Multinational Task Force. He is at a detention facility in Kuwait.

His alleged motive was unclear. He has been assigned a military attorney and has the option of hiring a civilian lawyer, authorities said.

The 42nd Infantry Division took over from the 1st Infantry Division in January and is responsible for a vast section of northern and central Iraq.

Martinez, who joined the National Guard in December 1990, was deployed to Iraq in May 2004.