Supporters of Army Pfc. Bradley Manning and WikiLeaks are objecting to the military's decision to close portions of Manning's hearing to the public and media on Monday.
Manning, 24, is accused of giving hundreds of thousands of classified documents to WikiLeaks to be published on the Internet.
The anti-secrecy organization has released raw field reports from Iraq and Afghanistan, diplomatic cables from U.S. embassies worldwide and video footage of a 2007 Apache helicopter strike that killed 12 in Baghdad.
Lt. Col. Paul Almanza, the investigating officer who is conducting the preliminary hearing at Fort Meade, emptied the courtroom on Monday to hear testimony involving classified information.
Among those excluded are an attorney for the Center for Constitutional Rights, which is representing WikiLeaks and its founder, Julian Assange.
Baher Azmy, legal director for the New York-based nonprofit, said the hearing has been "more restrictive than those at Guantanamo."
"We have a compelling interest in monitoring the hearing given both that Manning stands accused of providing evidence of war crimes to our clients, and that the proceedings are clearly closely linked to a grand jury in Virginia reported to be issuing subpoenas for information on our clients," Azmy said.
Manning supporters say the helicopter footage, in which the American helicopter crew can be heard laughing and referring to Iraqis as "dead bastards," appears to be evidence of a war crime. They say Manning is a hero who should be protected as a whistle-blower.
Monday is to be the fourth day of Manning's Article 32 hearing, and the third day of witness testimony. Almanza is hearing testimony and arguments before making recommending whether Manning's case should proceed to court-martial.
Manning is charged with aiding the enemy and violating the Espionage Act in the release of raw field reports from Iraq and Afghanistan and diplomatic cables from U.S. embassies, consulates and missions worldwide.
Aiding the enemy is a capital offense, but Army prosecutors have said they will not seek the death penalty. If convicted, Manning could be sentenced to life in prison.
In a statement Monday, the Bradley Manning Support Network said it was "deeply troubled" by "the imposition of an unexplained media blackout without any avenue for redress."
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"The investigating officer has already prevented Manning's defense from considering internal administration assessments that found these materials did not pose a threat to national security. Now he is seeking to prevent journalists and the public from reporting on testimony related to materials that are already in the public domain."
During testimony on Sunday, Special Agent David Shaver testified that he found thousands of classified files on Manning's computer. They included 10,000 State Department cables and video of the helicopter attack.
On Saturday, Capt. Steven Lim, one of Manning's superiors in Iraq, identified searches he believed the analyst conducted on government databases with keywords that included "WikiLeaks," "Julian Assange" and "Guantanamo Bay detainee assessments" — terms the Lim said Manning would not have needed for his regular intelligence duties in Iraq.
Manning's attorneys have sought to portray Manning as a troubled young man who struggled with gender identity disorder, was isolated from his fellow soldiers and should not have been given access to the classified materials.
Special Agent Toni Graham testified that Manning kept a folder of articles on gender identity disorder in his sleeping quarters, including one partially titled "flight into hypermasculinity." Special Agent Calder Robertson said Manning maintained an alter-ego called "Breanna Manning."
Lim confirmed details of an email Manning sent to an officer that included a picture of Manning dressed as a woman. The email included a plea that his confusion about his gender was preventing him from thinking clearly.
Lim testified that he did not learn of the email until after Manning was detained, but said its contents were alarming enough to merit removing his security clearance.
Lim also said Manning was disciplined for flipping a table, but was not removed from his job handling classified material.
"We needed the analysts to help us continue to work," Lim said.
Manning's team also has suggested that computer security at the facility in Iraq where he worked was lax, and rules were routinely broken.
Prosecutors have sought to emphasize that Manning was well trained in how to handle sensitive information and knew not to distribute it.