Michael Flynn, the controversial head of intelligence for the coalition in Afghanistan at the time, unintentionally violated intelligence regulations when serving as the Army's deputy chief of staff for intelligence, according to documents obtained by CNN from the Defense Department.
The military report was first obtained by The Washington Post.
Details are scant on what precisely Flynn did, but according to the report, he "inappropriately shared United States classified information with various foreign military officers and/or officials in Afghanistan."
The Army decided not to take any legal action against him, concluding that "there was no evidence of senior officer misconduct, rather there was keen attention to mission accomplishment in a coalition, combat environment."
The investigation "found that the sharing of classified information was not done knowingly and that there was no actual or potential damage to national security" due to Flynn's actions.
Nevertheless, the violation is yet another questionable mark on the resume of Flynn, who Trump has entrusted with a key intelligence posting despite a string of controversial comments and incidents.
Flynn's breach also comes after Trump's vociferous criticism of Democratic presidential nominee Hillary Clinton for her handling of sensitive information while serving as secretary of state.
CNN has reported that on at least two occasions, Flynn's handling of classified information came under scrutiny by the US military.
Two former government officials with direct knowledge of the issue told CNN that while Flynn oversaw intelligence in Afghanistan, he shared classified information with Pakistan on terror networks responsible for killing American troops. The intelligence, the sources said, came from another agency and Flynn wasn't supposed to share it. They said he was trying to convince Pakistan to stop sheltering terrorists.
In a separate incident, the two officials with whom CNN spoke said Flynn did not follow established security procedures when he shared classified intelligence with allies.
Flynn said he had permission to share the classified information. In both cases, sources said the retired general was informally reprimanded but never charged with wrongdoing.