James Dennehy, a US Marine Corps veteran who joined the bureau after the Sept. 11 attacks, announced his forced resignation in a letter to staffers at the FBI's New York Field Office.

"Late Friday, I was informed that I needed to put my retirement papers in today, which I just did. I was not given a reason for this decision," he wrote in the missive, obtained by The Post.

Dennehy apologized for the abruptness of his resignation in the letter, in which he called his colleagues "an office of professionals who will always do the right thing for the right reasons."