By Steve Gorman

(Reuters) - U.S. soldier Chelsea Manning, imprisoned for passing classified files to WikiLeaks, now stands accused of misconduct stemming from her suicide attempt earlier this month and could land in solitary confinement indefinitely, her lawyers said on Thursday.

The transgender Army private, who was born male but revealed after being convicted of espionage that she identifies as a woman, was notified by the Army on Thursday that she was under investigation, according to the American Civil Liberties Union.

The ACLU, which has acknowledged a suicide attempt by Manning earlier this month, said the 28-year-old soldier remains despondent over what the civil liberties group describes as the Army's continued denial of appropriate health care for her.

The ACLU denounced the latest disciplinary action as "unconscionable."

"While Chelsea is suffering the darkest depression she has experienced since her arrest, the government is taking actions to punish her for that pain," ACLU staff lawyer Chase Strangio said in a statement.

Pentagon and Army officials did not immediately reply to Reuters' requests for comment.

A transcript of the Army notice of investigation, as dictated over the phone by Manning to one her supporters and posted online by the ACLU, makes no explicit mention of a failed suicide.

But Manning was told the inquiry stemmed from her July 5 attempt to take her own life, which led to her being hospitalized for 24 hours, Strangio said.

The circumstances of the incident have not been disclosed, but the ACLU said it occurred in her cell and that she lost consciousness.

Manning has since been returned to confinement at the Disciplinary Barracks at Fort Leavenworth, Kansas, where she remains in a medical observation unit, Strangio said.

The notice lists three "administrative offenses" for which Manning is under investigation: "resisting the force cell move team," "prohibited property," and "conduct which threatens." Manning has yet to respond to the charges, Strangio said.

If convicted, she could face punishment that includes indefinite solitary confinement, reclassification into maximum security and an additional nine years in medium security, the ACLU said.

Manning, a former intelligence analyst in Iraq, was sentenced in 2013 to 35 years in prison after a military court conviction of providing more than 700,000 documents, videos, diplomatic cables and battlefield accounts to the anti-secrecy group WikiLeaks. The case ranked as the biggest breach of classified materials in U.S. history.

Among the files Manning leaked in 2010 was a gunsight video of a U.S. Apache helicopter firing on suspected Iraqi insurgents in 2007, an attack that killed a dozen people, including two Reuters news staff.

(Reporting by Steve Gorman in Los Angeles; Editing by Bernard Orr)