The Chicago Democrat has been on a secretive leave of absence for nearly seven weeks, during which his office has released only occasional snippets of information, including that he was undergoing treatment for a "mood disorder" at an undisclosed location.
On Friday, the Mayo Clinic distributed a statement from the congressman that said he had been transferred there for "extensive inpatient evaluation for depression and gastrointestinal issues." The clinic would not release more information Saturday.
John Anderson, of the Associates in Psychiatry and Psychology in southeastern Minnesota, said people receiving psychiatric care are often transferred to the Mayo Clinic when a physical illness develops because both can be treated there.
"Mayo does an excellent job in terms of combining those, so they can treat what's essentially a dual diagnosis," he said.
He noted specifically that Mayo Clinic has a highly rated gastrointestinal department as well as a free-standing inpatient psychiatric unit.
Jackson underwent a procedure in 2004 to help him lose weight that involves removing part of the stomach and rearranging the intestine. Phone messages left Saturday for Jackson's spokesman weren't immediately returned.
Friday's statement was the first to mention a gastrointestinal illness, and it was not known whether that was related to his depression or an entirely separate medical issue.
The statement did not disclose where the longtime Chicago congressman, the son of civil rights leader Jesse Jackson, had been staying previously.
He went on leave June 10, though his office didn't disclose it until weeks later and has been mum on details ever since. Initially, his office said Jackson was being treated for exhaustion. But his staff later said Jackson's condition was more serious and required inpatient medical treatment. They also said Jackson has been grappling with emotional issues.
Under mounting pressure from his colleagues and constituents, his office released a statement last month from his unnamed doctor saying Jackson was receiving intensive medical treatment at a residential treatment facility for "a mood disorder."
The timing of his medical leave has raised questions, in part because Jackson is facing an ethics investigation in the U.S. House connected to imprisoned former Illinois Gov. Rod Blagojevich.
The House Ethics Committee is investigating allegations that Jackson was involved in discussions about raising money for Blagojevich's campaign in exchange for the then-governor appointing him to President Barack Obama's vacated U.S. Senate seat. Blagojevich is serving a 14-year prison sentence for corruption.
Jackson was not charged and has repeatedly denied wrongdoing.
Jackson faces Republican and independent candidates in November, though he's widely expected to win re-election. He first won office in a 1995 special election and has easily won each race since. His district includes parts of Chicago and some suburbs.