A military judge, Col. Denise Lind, has ordered what prosecutors say is an unprecedented closed hearing Wednesday at Fort Meade to help her decide how much of Manning's upcoming trial should be closed to protect national security.
An unidentified prosecution witness will testify during that closed hearing in a "dry run".
Defence attorneys say that could allow the judge to find ways to avoid closing the courtroom to the public during the presentation of classified evidence.
The sensitive evidence includes Iraq and Afghanistan battlefield reports and State Department cables Manning has acknowledged leaking, along with official communications about those classified documents.
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The government says the leaks in 2009 and 2010 endangered lives and security. Manning's lawyers contend there was little to no damage.
Lind's decision to hold the practice run out of public view has drawn mixed reactions from national security and legal experts.
But Jesselyn Radack, national security and human rights director of the Washington-based Government Accountability Project, said there has already been too much secrecy in the Manning case.
Until February, more than two years after his arrest, the military refused to publicly release written court filings and rulings in the case.
Manning is charged with 22 offences, including Espionage Act violations and aiding the enemy, which carries a possible life sentence. He pleaded guilty in February to reduced charges that could send him to prison for 20 years, but military prosecutors said they would still try to convict him of the greater offences.