The leak of classified information was a "deliberate, criminal act," the Pentagon said on Thursday.
Pentagon spokesman Brigadier General Pat Ryder referred questions about the investigation to the Department of Justice.
Ryder added the Pentagon had taken steps to review distribution lists and ensure that people receiving information had a need to know.
FBI says it has made arrest in Massachusetts, amid a report leaker identified
The FBI said on Thursday it has "made an arrest and is continuing to conduct authorized law enforcement activity at a residence in North Dighton, Massachusetts," amid a media report a suspect in the leak of classified has been identified.
The United States arrested Air National Guard employee Jack Teixeira on Thursday in connection with transmission of classified defense information, US Attorney General Merrick Garland said.
"FBI agents took Teixeira into custody earlier this afternoon without incident," Garland said at a brief statement at the Justice Department.