Rep. Jesse Jackson Jr.'s strange absence from Congress is due to clinical depression and a stay in an Arizona substance abuse clinic, where he was being treated for alcoholism, NBC's Nightly News reported on Wednesday night.

Speculation has swirled in recent weeks that marital problems and stress from a congressional ethics investigation stemming from the conviction of former Illinois Governor Rod Blagojevich (D) had become unbearable for Jackson, ultimately forcing him to seek treatment.

His office called the Congressman's condition a "mood disorder," disputing the claim of alcoholism and marital discord. Aides to Jackson claimed in June that he had taken a leave for "exhaustion."

"This is about a human being who’s sick," Rep. Emanuel Cleaver (D-MO) told The Washington Post on Wednesday. "This is not a political matter, it’s a health matter."

Jackson went on medical leave on June 10, but his office did not announce his absence until two weeks later.

Speaking to a Chicago news station on Tuesday, Jackson's father, the civil rights icon Rev. Jesse Jackson, explained that his son is "taking his time in recovery."

"I speak as a father, an one who right now is pained as he comes out of his crisis," he said. "We're with him and we hope that he'll be fully restored to his health. But right now he's going through a tremendous challenge."

One of Jackson's former fundraisers, Raghuveer Nyak, was arrested days after Jackson's medical leave began. Audio from the trial of Gov. Blagojevich revealed that the convicted Democrat claimed that Nyak had offered to raise $1.5 million in campaign donations if Blagojevich would pick Jackson to be the next Senator from Illinois.

Nyak's arrest does not appear to be connected to Jackson. He faces charges of mail fraud, racketeering and bribing doctors. Jackson has denied involvement in the Blagojevich scandal.

This video was broadcast by NBC's Nightly News on Wednesday, July 12, 2012.

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