Rep. Jesse Jackson resigned from Congress Wednesday, saying in a letter that he is cooperating with a federal investigation "into my activities" but blaming his health problems for his decision to step down just two weeks after his re-election. "I 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," Jackson said in the two-page letter dated Nov. 21. "None of us is immune from our share of shortcomings or human frailties and I pray that I will be remembered for what I did right." The announcement comes two weeks after Jackson won re-election without campaigning. The 47-year-old Jackson had not been on Capitol Hill since late May. He has been on medical leave for bipolar depression.