The person shared classified information to a group on the instant messaging platform Discord of about two dozen men and young boys who shared a "mutual love of guns, military gear and God", the Post said.
The Post based its report, which did not name the person, on interviews with two members of the Discord chat group.
Reuters was unable to verify details of the report.
Discord said in a statement earlier on Wednesday that it was co-operating with law enforcement.
The Department of Defense and the Department of Justice did not immediately respond to a Reuters request for comment.
The Department of Justice opened a formal criminal probe last week after the matter was referred by the Pentagon, which is assessing the damage done by what may be the most damaging release of classified US information in years.
The person went by the handle OG, slang for Original Gangster, or an old school traditionalist. The person was described by one of the Post's sources as being in his early to mid-20s, and was looked up to by members of the group.
"He's fit. He's strong. He's armed. He's trained. Just about everything you can expect out of some sort of crazy movie," said one member of the chat group, who was under 18 and spoke on the condition of anonymity with the permission of his mother, the Post reported.
In what appears to be the gravest leak of US secrets in years, pictures of sensitive documents were posed on Discord and other platforms including the online messaging board 4Chan, the encrypted Telegram global messaging app, and Twitter.
US national security agencies and the Justice Department are investigating the release to assess the damage to national security and relations with allies and other countries, including Ukraine.
