WASHINGTON—Pentagon lawyers believe that online whistleblower group WikiLeaks acted illegally in disclosing thousands of classified Afghanistan war reports and other material, and federal prosecutors are exploring possible criminal charges, officials familiar with the matter said.

A joint investigation by the Army and the Federal Bureau of Investigation is still in its early stages and it is unclear what course the Department of Justice will decide to take, according to a U.S. law-enforcement official.

He said WikiLeaks founder Julian Assange had not been identified by the FBI as a target of the probe.

WikiLeaks in late July posted on its website some 76,000 classified military documents, the largest such disclosure since the release of the Pentagon Papers in 1971. It has promised to publish another 15,000 documents from the cache it obtained. The disclosure infuriated the Pentagon, which warned that the release could endanger allies in Afghanistan and undercut the war effort.

Several officials said the Defense and Justice departments were now exploring legal options for prosecuting Mr. Assange and others involved on grounds they encouraged the theft of government property.

Bringing a case against WikiLeaks would be controversial and complicated, and would expose the Obama administration to criticism for pursuing not just government leakers, but organizations that disseminate their information.

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