WASHINGTON — The Army filed new charges Friday against the U.S. soldier accused of killing 16 villagers in southern Afghanistan in March, alleging that he had consumed alcohol and used steroids during his combat tour.

According to the charges, Staff Sgt. Robert Bales illegally possessed and used stanozolol, an anabolic steroid that is commonly used by bodybuilders but is banned by many professional sports leagues as a performance-enhancing drug.

The Army did not specify how much of the drug Bales may have used or when, except to say that the use occurred between Jan. 1 and March 11. It was in the early morning hours of March 11 when he allegedly shot and killed civilians in the village of Belambay in Kandahar province in what is considered the single worst U.S. atrocity of the Afghan war.

It is the first time that military officials have indicated steroids might have played a role in the killings. The Army has not disclosed an alleged motive for the murders. Bales’s attorneys are contesting the evidence and have not acknowledged any wrongdoing on his part.

Some U.S. officials, speaking on condition of anonymity, had previously suggested that alcohol might have been a factor, saying that there was evidence Bales had been drinking before the slayings.

The court documents released Friday said he drank sometime between Nov. 1 and March 10 but did not give details. U.S. troops are prohibited from drinking in Afghanistan to avoid offending its conservative Muslim population.

Emma Scanlan, a civilian attorney from Seattle who is part of Bales’s defense team, downplayed the steroid and alcohol charges. She said that, despite the official prohibitions against both substances, it was common for soldiers to use steroids and to drink on remote outposts like the one where Bales was deployed.

“The access to alcohol and other substances appears to be fairly widespread,” she said.

Scanlan said Army investigators on Friday delivered to the defense team 5,000 pages of evidence and other material to support the charges against Bales. She said she was still reviewing the material.