An accountability nonprofit group on Tuesday called for the Pentagon's top watchdog to investigate Defense Secretary Ash Carter's use of personal emails, calling it a "flagrant disregard for following rules."

The Campaign for Accountability asked the Defense Department inspector general in a letter to look into Carter's use of personal email, which was revealed last year.

Carter said immediately after the reports that it was a "mistake" to use his personal email for official business, yet he continued the practice for at least two months after news broke that former Secretary of State Hillary had been using her personal email for work business.

Anne Weismann, executive director of the Campaign for Accountability, said Carter's use of personal email raises questions of whether sensitive information was compromised and whether emails sent from his personal account were captured for the federal record that's accessible to the public.

"Secretary Carter, as the top official at the Pentagon, has access to some of the most sensitive information in the government," Weismann said. "By using his personal email, he could have put government secrets at risk. The inspector general should immediately open an investigation into whether Secretary Carter violated any federal laws or DoD policies."

The Senate Armed Services Committee announced late last year that it would investigate Carter's use of personal email to ensure it didn't compromise any classified information.

"With all the public attention surrounding the improper use of personal email by other administration officials, it is hard to believe that Secretary Carter would exercise the same error in judgment," Sen. John McCain, R-Ariz., said in a statement.