D.C. Council member Michael A. Brown said Tuesday he expects investigators to unravel how more than $100,000 went missing from his campaign treasury over the past year.

In a jolting disclosure to the city’s Office of Campaign Finance (OCF), Brown reported late Monday night that he needed to adjust his campaign reports to account for $113,950 in “unexplained expenditures” that nearly wiped out his campaign account. The loss left him with about $18,000 in the bank as of Aug. 10.

Brown (I-At Large), the council’s president pro-tempore, amended his finance reports 2 1/2 months after he first alerted authorities and the public that he believed a “substantial” amount of money had been stolen.

After initially saying little about what he calls “embezzlement,” Brown made a brief appearance before reporters in front of the John A. Wilson Building on Tuesday afternoon.

“Frankly, I don’t believe being a victim of a crime reflects badly on the campaign,” said Brown. “I think it reflects badly on the thief. . . . I feel profoundly betrayed.”

Brown’s allegations of what election observers say would be one of the largest thefts from a city political campaign is the latest blow to the image of a District government reeling from an ongoing federal investigation into Mayor Vincent C. Gray’s (D) 2010 election effort. Over the past 10 months, two D.C. Council members, including the former council chairman, have been forced from office after being charged with federal crimes.

Brown’s campaign troubles surfaced just as he’s gearing up for a race to return to a D.C. Council seat reserved for a non-Democrat.

One of Brown’s opponents, Mary Brooks Beatty (R), called on Brown to withdraw his candidacy. “Throughout his time in office, Council member Brown has demonstrated a loose commitment to professional ethics and accountability,” Beatty said in a statement. “The people of Washington D.C. need an election focused on making the District a better place to live and not the campaign problems of Council member Michael Brown.”

The son of late commerce secretary Ron Brown, Michael Brown joined the council in 2009 and was regarded as a potential mayoral candidate. But Brown has struggled with his personal finances, including failure to pay his rent, mortgage and taxes on time.

Also, after his 2008 campaign, an OCF audit found that Brown’s campaign had written nearly $81,000 in checks that were returned because of insufficient funds, failed to report $121,000 in expenditures, and wrote seven checks worth a total of $16,042 payable to “cash” in violation of city rules.

Brown said he discovered that the 2012 campaign funds were missing after reviewing bank statements this summer. He dismissed his campaign treasurer, Hakim Sutton, and called police. Sutton has declined comment, and did not return messages on Tuesday.

Gwendolyn Crump, a police spokesman, said Tuesday that D.C. police and the U.S. Attorney’s Office are conducting “a very active investigation.” OCF is also investigating.

OCF initially said Brown did not have to disclose how much money was missing while the office conducted its investigation. But Brown was forced to update his filings after council candidate David Grosso (I) demanded that the Board of Elections require Brown to more accurately account for the missing funds. Election officials eventually ordered Brown to file an amended report.

In his revised filings for March 11 through June 10, Brown lists 34 new “unexplained expenditures” totaling $113,950. Some of the expenses date to July 2011. The report states that the expenditures were made by Sutton.

“We have satisfied all old requests from the Office of Campaign Finance to the highest ethical standards of transparency,” said Brown, who walked away from the Wilson Building news conference before most reporters could question him.

Brown wrote to Cecily E. Collier-Montgomery, director of OCF, on Monday stating he has turned over copies of bank statements and other documents to D.C. police and auditors.

“I have been assured by the MPD and the United States Attorney’s Office that they believe they have a strong case against the perpetrator of the theft and that they are actively pursuing it,” Brown wrote. “Some type of law enforcement action is expected in the coming weeks against the individual responsible for the theft.”

With the Nov. 6 election less than two months away, the loss of campaign funds could hamper Brown’s ability to compete for a second term. According to campaign finance records, he has less than one-third the amount that Grosso reported as of Aug. 10. Brown also now has less campaign funds in the bank than Beatty.

The loss also could intensify questions about Brown’s leadership and fiscal stewardship, including a $50,000 IRS lien and late payments on his mortgage, D.C. property taxes, and an apartment he rents in Northwest.

Brown’s 2012 campaign has also been hobbled by a challenge this month from Grosso and activist Dorothy Brizill over whether he collected enough valid signatures to qualify for the November ballot.

Last month, Brown submitted 4,675 signatures to the elections board to meet city requirements that at-large council candidates collect at least 3,000 signatures from registered voters. But after Grosso and Brizill alleged that some of the signatures were forged or improperly gathered, the elections board rejected more than 1,500 of Brown’s signatures as duplicative or from unregistered voters or residents whose addresses could not be verified.

The board ruled Monday that Brown was left with 3,166 valid signatures, barely enough to qualify for the ballot.

Mike DeBonis contributed to this report.