The U.S. Army on Friday dropped one of the murder charges against the soldier originally accused of killing 17 Afghan villagers in March, reducing the number of murder counts to 16.
The amended complaint now also accuses Staff Sergeant Robert Bales of wrongfully possessing and using steroids and unlawfully consuming alcohol while deployed.
Bales, a decorated veteran of four combat tours in Iraq and Afghanistan, was initially charged with killing eight adults and nine children and with six counts each of assault and attempted murder for attacking two other adults and four children.
The Army said in a statement the amended charges against Bales stemmed from "developments in the ongoing investigation" into the March 11 incident in Afghanistan's Kandahar province. The statement did not elaborate.
Bales is accused of walking off his base under cover of darkness and opening fire at civilians in their homes in at least two villages in Panjwai district.
The mass shooting further eroded U.S.-Afghan relations, already frayed by a decade of war.
Initial reports from Afghanistan put the death toll at 16 people, despite the Army's original decision to charge Bales with 17 counts of murder.
A Seattle-based attorney for Bales, John Henry Browne, could not immediately be reached for comment.
The six counts of attempted murder against Bales remain unchanged in the amended complaint. But the newly filed documents also add a count of assault, which brings the total number of those counts to seven, from the previous six.
The new allegation involves an incident from February, the month before the mass shooting, in which Bales is accused of using his hands and knees to "unlawfully strike" a male Afghan whose name is unknown, according to the charging documents.
Bales is being held at Leavenworth military prison in Kansas, but was assigned to the 2nd Battalion, 3rd Infantry Regiment headquartered at Joint Base Lewis-McChord in Washington state.
Premeditated murder is a capital offense under the U.S. military justice code, so Bales could face the death penalty if convicted.
He would face a mandatory minimum sentence, if convicted, of life imprisonment with eligibility for parole, the military has said.