WASHINGTON—Secretary of War Pete Hegseth used Signal to send sensitive, nonpublic operational information that he determined did not require classification surrounding U.S. strikes on Yemen, a Defense Department watchdog concluded.

The department’s Office of Inspector General noted in a report dated Dec. 2 and released on Dec. 4 that Hegseth makes the determination of what is or is not classified.

Hegseth took “non-specific general details” that he determined, as an original classification authority, were either not classified or that he could safely declassify and use to create an “unclassified summary” for the Signal chat participants, according to the report.

However, Hegseth did not comply with department protocol when using his personal cellphone to conduct official business, the report said.

Hegseth and top administration officials used a group chat on Signal, titled “Houthi PC Small Group,” to communicate during the March 15 strikes on Yemen. The Atlantic published two reports on the group chat as its editor-in-chief, Jeffrey Goldberg, was added to it.

The report from the Office of Inspector General found that Hegseth could have jeopardized the mission and the safety of the military.

“The Secretary sent nonpublic [Defense Department] information identifying the quantity and strike times of manned U.S. aircraft over hostile territory over an unapproved, unsecure network approximately 2 to 4 hours before the execution of those strikes,” the report stated. “Using a personal cell phone to conduct official business and send nonpublic [Defense Department] information through Signal risks potential compromise of sensitive [Defense Department] information, which could cause harm to [Defense Department] personnel and mission objectives.”

The Office of Inspector General requested from the Defense Department a copy of Hegseth’s communications on Signal. The department provided a partial copy of the messages, as some messages were automatically deleted because of the chat settings. The office partially relied on the transcript that The Atlantic published.

Hegseth declined to be interviewed by the Office of Inspector General for its report. He instead sent a written statement to the office that included a confirmation that he sent operational information about the Yemen mission to members of the group chat.

The Office of Inspector General recommended that the Defense Department improve its training of senior officials on using electronic devices.

The report is a result of Sens. Roger Wicker (R-Miss.) and Jack Reed (D-R.I.)—the chairman and a ranking member of the Senate Armed Services Committee, respectively—requesting on March 26 that the inspector general probe Hegseth’s use of Signal for official business.

The Defense Department celebrated the report.

“No classified information. Total exoneration. Case closed. Houthis bombed into submission. Thank you for your attention to this IG report,” Hegseth posted on X.