Matt Bissonnette could face charges for publishing classified information in his book "No Easy Day," The Washington Post reported.
The Pentagon released a letter Thursday night from Defense Department legal counsel Jeh C. Johnson which accused Bissonnette of being in "material breach and violation" of agreements he signed in 2007.
In those documents, the 14-year Navy veteran promised not to disclose classified information.
Johnson did not say what the specific violations are.
The book is due to go on sale next week.
Bissonnette, who wrote under the pen name Mark Owen, says in the book he hired a former Special Operations attorney to review the manuscript for "forbidden topics."
The Pentagon said earlier it would defer to the Justice Department about whether any charges would be filed against the author.
Bissonnette retired from the service in April.