The Pentagon on Thursday warned a former Navy SEAL that it was considering taking legal action against him for publishing a first-hand account of the raid that killed Osama bin Laden, implying that he had divulged classified information.

It remained unclear, however, whether the Defense Department’s warning to Matt Bissonnette, author of the highly publicized “No Easy Day,” was a precursor to a legal battle with the ex-commando or an empty threat.

After taking several days to read Bissonnette’s manuscript, the Pentagon waited until 8 p.m. Thursday — the final night of the Republican National Convention — to release a copy of a letter from Jeh C. Johnson, the Defense Department’s legal counsel, reprimanding the author for writing the book without first clearing it with the military.

In the letter, Johnson formally notified Bissonnette that he was in “material breach and violation” of two agreements he signed in 2007 promising not to disclose classified information.

Johnson was not more specific and did not cite any examples. The letter was addressed to “Mark Owen,” the pen name adopted by the former SEAL in an attempt to protect his identity. Bissonnette was identified last week by Fox News as the author of the book. He resigned from the Navy in April after 14 years of service.

In an author’s note to the book, which is scheduled to go on sale next week, Bissonnette insisted that he would not reveal any classified information and said he hired a former Special Operations attorney to review the manuscript and “ensure that it was free from mention of forbidden topics.”

Until Thursday night, the Pentagon and the Obama administration had refrained from criticizing Bissonnette directly. Officials have not commented on his descriptions of the bin Laden raid, which contradict accounts given by the White House in some key respects, including the circumstances under which the al-Qaeda leader was shot and killed.

The Pentagon earlier said it would defer to the Justice Department on whether a criminal case against Bissonnette was merited.

Lt. Col. Steven H. Warren, a Pentagon spokesman, said defense officials would have no further comment on the letter. “It stands for itself — there is nothing to add at this time,” he said in a statement released Thursday night.

It seems doubtful, however, that the Obama administration would decide to prosecute a man who is widely seen as a national hero for his role in killing bin Laden.

The administration is also hamstrung because it encouraged a pro-Obama Hollywood producer to make a movie about the bin Laden raid.