"Against the recommendations of my doctors, I had hoped and tried to return to Washington and continue working on the issues that matter most to the people of the Second District," he wrote. "I know now that will not be possible."

Jackson was also being investigated for allegedly misusing campaign funds, and his letter referred to that as well.

"I am aware of the ongoing federal investigation into my activities and am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone," he wrote.

Jackson's resignation means there are currently 433 members of the House. The other open seat was held by Rep. Dennis Cardoza (D-Calif.), who retired in August and is now at Manatt, Phelps & Phillips law firm.

As of Tuesday evening, Jackson's House website was still in working order, although his last press release was issued on June 19, just before he sought medical treatment.