A congressional committee has launched an official probe into allegations by Sulaimon Brown, the fired D.C. government employee and former mayoral candidate who says that Mayor Vincent C. Gray promised him a job and that he received payments from two members of Gray’s campaign for attacks on then-Mayor Adrian M. Fenty last year.

Last week, staffers of the House Committee on Oversight and Government Reform reached out to Brown and others to learn more about the process city agencies were planning to use to investigate the allegations. But they were struck by the lack of cooperation from Gray campaign chairman Lorraine Green, campaign consultant Howard Brooks and Gerri Mason Hall, who was ousted Wednesday as Gray’s chief of staff, said committee spokesman Frederick R. Hill.

“It’s unprecedented, the lack of information we’ve received,” Hill said in an interview Thursday.

An investigator for the committee, chaired by Rep. Darrell Issa (R-Calif.), said he has obtained evidence that Brown did not interview for his $110,000 job as a special assistant in the Department of Health Care Finance. He was simply placed there, Hill said.

The committee has subpoena power and could hold public hearings on the matter, Hill said.

“We haven’t ruled out hearings,” he said.

The committee’s action adds another layer of investigation outside D.C. government. The U.S. attorney’s office previously announced that it “is working with the FBI to assess the matter.”

In a statement, the mayor’s office said there are “sufficient investigative bodies” looking into the allegations. “Congressional involvement is not likely to illuminate any additional issues or information,” it said. “Although the mayor, in his support for the autonomy of the District of Columbia, does not encourage congressional oversight of city matters, he and his administration will cooperate fully with this investigation.”

Del. Eleanor Holmes Norton (D-D.C.), said in a statement: “A congressional investigation would take the new House Republican majority’s obsession with invading the District’s home rule to a new low. I hope to meet with Chairman Issa to ask him to defer to the investigations now underway.”

Brown, 40, an unemployed auditor who has a limited resume and has had six run-ins with the law through the years, told The Washington Post that he landed the six-figure job in return for bashing Fenty during the campaign. He said Green and Brooks gave him payments to keep his campaign afloat. The Washington Post could not independently verify any payments. But Brown exchanged text messages with Gray, and Brown’s phone records show several exchanges with numbers belonging to Gray, Green, Brooks and a personal campaign aide to the mayor.

Green, Brooks and Gray have denied Brown’s allegations. Gray has said that he promised Brown only a job interview. Green has said she told Brown that he would have to “qualify” for a job.

“I’m pleased that the Congress oversight committee is taking the allegations seriously, because this is serious,” said Brown, who met with the FBI last week and said he is scheduled to meet again Friday. He would not comment further.

Brown was hired Jan. 31. On Thursday, Wayne Turnage, the department’s director, told reporters that Brown was employed in the office when he arrived Feb. 1 and that he met with Brown on Feb. 10. He said Hall told him that he could decide whether Brown should stay. He said that based on Brown’s qualifications, he decided he would be appropriate for “back-office operations.”

Turnage said personnel rules prohibited him from revealing why he fired Brown on Feb. 24, but he said that “it was my decision to fire him.”

On Monday, Issa sent letters to attorneys for Green, Brooks and Hall, telling them that the panel is investigating and that it wants to interview their clients. Issa said in the letter that the committee is the House’s “principal oversight committee” and has “special jurisdiction” over D.C. government.

In a statement Thursday regarding the committee probe, Issa said his investigators’ “initial findings . . . do not give me confidence that the District government can make this evaluation.”

The committee has not reached out directly to Gray for an interview, Hill said.

Within the D.C. government, the Office of Campaign Finance and the Office of the Inspector General have announced investigations.

The Office of the Attorney General announced that it will “facilitate” those probes.

Inspector General Charles Willoughby later said that his office would not investigate, however, because he had met with Brown about a job in the agency that had already been filled.

Green has retained Thomas C. Green, a lawyer whose biography on the Web site of Sidley Austin highlights his representation of one of the defendants in the Watergate probe.

Glenn F. Ivey, former Prince George’s County state’s attorney, is representing Brooks. Hall has retained Kenneth Wainstein, former U.S. attorney for the District who represented former Washington Wizards player Gilbert Arenas on his 2009 weapons charge.

“We are confident that when these reviews are over, the conclusion will be that Mr. Brooks did not break the law,” Ivey said in a statement.

Through her attorney, Hall canceled a Tuesday interview with the committee’s staff, Hill said.

Wainstein, who is out of the country, did not immediately return calls for comment.

Thomas Green, no relation to his client, said he was aware of the committee’s probe.

“There’s a lot of investigations going on,” he said. “We’re in the process of deciding how we will respond to these requests, and we will proceed in due course.”