Retired general quits as president of Brookings Institution amid FBI probe of his possible illegal lobbying for Qatar

The president of the Brookings Institution resigned Sunday amid a federal investigation into whether he illegally lobbied on behalf of the wealthy Persian Gulf nation of Qatar.

Retired Gen. John Allen wrote in a letter to the think tank that he was leaving with a "heavy heart" but did not offer a direct explanation.

"I know it is best for all concerned in this moment," Allen's letter said.