Mitch Dumke / Reuters file

Rep. Jesse Jackson Jr., D-Ill.

Rep. Jesse Jackson Jr., D-Ill., returned to Washington, D.C., Friday after nearly three months away from his job.

"I have just learned that Congressman Jackson is at home in DC with his wife and family convalescing," his communications director, Frank Watkins, said Friday.

Jackson, 47, suddenly left his post  June 10 and checked into an Arizona substance abuse clinic. He represents Illinois' 2nd Congressional District. About a month later he checked into the Mayo Clinic.

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Asked if Jackson would be back at work Monday, his chief of staff, Rick Bryant, told NBC News: "I hope so."

The Mayo Clinic revealed last month that Jackson, the son of civil rights leader and former presidential candidate Jesse Jackson, was being treated for bipolar depression, a condition that affects the parts of the brain controlling emotion, thought and drive.

Millions of people have bipolar disorder. It is marked by highs and lows of mood, and can be treated by medication and psychological counseling, according to the Mayo Clinic's website.

It's not clear when Jackson checked out of the clinic. Doctors there would not give specifics and referred all questions to his congressional office.

Jackson faces a reelection battle this November.

This article includes reporting by NBCChicago.com, Reuters and NBC News' Nadine Comerford. Follow her on Twitter: @NComerfordTV

Related: Patrick Kennedy has 'emotional' meeting with Jesse Jackson Jr.

Mitch Dumke / Reuters

In this file photo, U.S. Rep. Jesse Jackson Jr. speaks at a news conference on Capitol Hill in Washington on Dec. 10, 2008. The congressman is currently on medical leave.

Rep. Jesse Jackson Jr. is "regaining his strength," following his admittance to the Mayo Clinic for bipolar disorder earlier this summer, the Rochester, Minn., facility said Monday.

The Illinois Democrat, who has been on medical leave from his job since June 10, is being treated for Bipolar II depression, the clinic said. Jackson has been at the Mayo Clinic since July 25.

"Congressman Jackson is responding well to the treatment," Mayo Clinic spokeswoman Traci Klein said in a statement. Jackson asked the clinic to distribute this information on his behalf, and Monday's update comes after mounting pressure to reveal the nature of his medical leave.

The Mayo Clinic says Bipolar II disorder is a treatable condition, affecting parts of the brain that control drive, thought and emotion. This type of depression is most likely caused by a set of genetic and environmental factors.

Jackson's wife said earlier this summer that the congressman was "completely debilitated" by depression.

Klein said the 47-year-old congressman underwent gastric bypass surgery back in 2004, which changes how the body absorbs things like foods, liquids and medications.

There's no timeline for Jackson's recovery, according to Jackson's father, civil rights activist Rev. Jesse Jackson. The elder Jackson said earlier this month that his son has no plans to step down from his post.

Jackson is currently under investigation by the House Ethics Committee for his connections to former Illinois Gov. Rod Blagojevich, who was convicted on corruption charges for trying to sell off Barack Obama's Senate seat after Obama was elected president. Jackson has repeatedly denied wrongdoing in the scandal and has not been charged.

Former Rhode Island Rep. Patrick Kennedy said Monday he plans to visit Jackson on Thursday at the Mayo Clinic, The Associated Press reported. Kennedy was treated at the Mayo Clinic himself for addiction and depression in 2006 after a late-night car crash at the U.S. Capitol. He decided not to run for a ninth term in 2010 and retired from Congress last year. 

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Mitch Dumke / Reuters

U.S. Rep Jesse Jackson Jr., D-Ill., has been on an extended leave of absence from his job while he gets treatment at the Mayo Clinic.

The wife of Rep. Jesse Jackson Jr. says he is “completely debilitated” by depression but Jackson’s father says the Illinois congressman has no plans to quit his post.

The Rev. Jesse Jackson said he has visited his son at the Mayo Clinic in Rochester, Minn., where he is being treated. He said there’s no timeline for his recovery.

“He’s taking his treatment. This takes time. We can’t put a timetable on it.,” the elder Jackson said Saturday after his weekly radio broadcast at Rainbow PUSH, the Chicago Sun-Times reported. 

The elder Jackson spoke a day after his daughter-in-law, Sandi Jackson, told the Sun-Times her husband had become “completely debilitated by depression.” 

Sandi Jackson, an alderman for Chicago’s 7^th Ward, denied rumors that her husband attempted suicide or was being treated for alcohol and drug addiction.

““No, no, none of that is true,” she told the Sun-Times.

The fate of Jesse Jackson Jr., an Illinois Democrat, has been the subject of speculation ever since he reportedly “collapsed” on June 10 at his home in Washington. He sought treatment for exhaustion at the Sierra Tucson Treatment Center in Arizona, then transferred to the Mayo Clinic.

Jackson and his family have released few details of his ordeal.

 “I fully expect him to return to work, but not a day before the doctor says it’s OK,” Sandi Jackson told the Sun-Times. “And although he has talked about teaching, he’s focusing on going back to work, back to his job. He is a gifted legislator.”

Jackson’s illness comes as the House Ethics Committee is investigating his connections to former Illinois Gov. Rod Blagojevich, who was convicted on corruption charges in connection with his efforts to profit from appointing a successor to the Senate seat vacated by Barack Obama after he was elected president.. Jackson was not charged and has repeatedly denied wrongdoing.

Mitch Dumke / REUTERS

Rep. Jesse Jackson Jr. had been treated at an undisclosed location in Arizona for an undisclosed illness.

Updated at 11:50 p.m. ET: Rep. Jesse Jackson Jr., struggling with an undefined illness in an undisclosed location for nearly a month, has left treatment in Arizona and has entered the world-renowned Mayo Clinic.

In a statement released through the Rochester, Minn. facility, Jackson said he went in for "extensive inpatient evaluation for depression and gastrointestinal issues."

"Congressman Jackson and his family are grateful for the outpouring of support and prayers that have been received throughout his care," reads a portion of the statement.

See the original report |  More from NBCChicago.com 

Further information would be released as his evaluation proceeds, it said.

Jackson in 2004 underwent gastric bypass surgery, though he kept the procedure a secret for several months.

His absence began in late June with an announcement that he was being treated for exhaustion. Since then only the barest of details have been released.

About a week later, Jackson's office issued a statement noting the congressman's condition is "more serious than we thought and initially believed." Another week went by before an email attributed to Jackson's doctor said the congressman is suffering from "mood disorder."

The doctor noted he is responding positively to "intensive medical treatment at a residential treatment facility."

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The congressman’s office added that rumors about Jackson Jr. being treated for alcohol or substance abuse "are not true," but a family friend told NBC News that he has severe clinical depression, a drinking problem or a problem with alcohol and was undergoing treatment in Arizona.

Since his absence began, many have called for more information about Jackson's whereabouts and ailment.

“I think Congressman Jackson and his office and his family would be well advised to advise his constituents of his condition," said Democratic Congressman Steny Hoyer a day before news of Jackson's mood disorder surfaced. "He’s obviously facing a health problem."

"[Jackson’s] health is a number one priority,” Sen. Dick Durbin said. "As a public official though, there reaches a point where you have a public responsibility to tell people what you’re facing and how things are going.”

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Gov. Pat Quinn, however, has said he would not call for more information to be released.

"The people of Illinois have good hearts," Quinn said July 11. "I pray for Jesse Jackson Jr. every day."

Jackson's family remains mum on the congressman's exact ailment. His father, Rev. Jesse Jackson, said he is "hopeful" for recovery.

"As a father, I offer no medical diagnosis, only the unconditional love of his family," he said.