Prosecutors sought to emphasize that Manning, the 24-year-old accused of sending hundreds of thousands of classified files to the anti-secrecy group WikiLeaks, was well trained in how to handle sensitive information and knew not to distribute it.
Manning's direct supervisor, Sgt. First Class Paul Adkins, was set to testify Sunday but invoked his Article 31 rights, similar to the Fifth Amendment protection against self-incrimination.
Sunday was the third day of a military hearing at Fort Meade to determine whether Manning's case should go to court-martial. Manning is charged with aiding the enemy and violating the Espionage Act, and could face life in prison.
Defense attorneys have tried to argue that superiors such as Adkins should have recognized Manning's emotional problems and cut off his access to classified information. According to previous testimony, Manning had emailed Adkins saying he was confused about his gender identity and sent him a picture of himself dressed as a woman.
On Sunday, Special Agent David Shaver, who investigated Manning, testified that he found thousands of classified files on Manning's computer. They included 10,000 State Department cables and video of an Apache helicopter attack on civilians, which was released by WikiLeaks.
Much of Sunday's testimony focused on security protocol for the military's computer networks.
Soldiers routinely stored music and games on a network used to store classified information, witnesses said. They also played pirated movies bought from Iraqis on the computers, according to the testimony.
Capt. Thomas Cherepko, who managed the computer network at Manning's base, said he didn't know of anyone being disciplined for putting unauthorized programs on the classified computer drive, even though that was against the rules.
Cherepko also acknowledged that he had received a letter of admonishment in March for failing to accredit and certify the network.
Manning, dressed in camouflage fatigues, sat calmly with his lawyers in the courtroom. At times, he leaned forward to turn off a microphone at his table so he could confer with the attorneys.
Warrant Officer 1 Kyle J. Balonek, who also supervised Manning, also invoked his right to not testify Sunday.
The presiding officer in the hearing, Lt. Col. Paul Almanza, will recommend whether the case will proceed to court-martial. Given the amount of evidence expected in the case, it could be weeks before a decision is made, a legal expert with the Army said Sunday.
While much of the hearing Sunday highlighted technical aspects of the computer networks that store secret information, some witnesses also provided glimpses of Manning's behavior while deployed.
Capt. Casey Fulton, the prosecution's first witness Sunday, said she often turned to Manning to pull together intelligence from a variety of sources because "he had a better understanding than any of the other analysts."
"He was very good at compiling data," she said.
But Fulton said that after Manning was involved in an altercation with a female soldier named Spc. Jihrleah Showman, she recommended that he have his weapon taken away and be removed from the computer center.
Fulton said that Manning and other analysts were trained on how to handle secret information, but said they were not supervised at all times.
"There's only a limited amount of supervisors," she said. "You can't supervise everyone every second of the day."
Fulton also described how she and Manning discussed the Apache helicopter video, which Manning is accused of sending to WikiLeaks. After the video went public, Manning sent Fulton an email showing two video clips — one from the military's computer drive, and the other from WikiLeaks.
Sgt. Chad Madaras, who shared a work station with Manning in the computer room, described Manning as an outcast and said he saw several emotional outbursts.
"He kind of separated himself from others in the unit," Madaras said in response to questions from defense attorney David Coombs.
Almanza ruled Sunday that part of Special Agent Shaver's testimony contains classified information, and said he would close part of the hearing Monday morning to hear the evidence.
The defense objected to keeping the testimony closed.
On Saturday — Manning's 24th birthday — defense lawyers portrayed the soldier as a troubled young man who struggled with gender identity issues and who should not have been given access to the classified materials. A witness revealed that Manning had an alter ego named "Breanna." Another said he had few friends and kept a folder of articles on gender identity disorder in his sleeping quarters.