In this April 2010 image from video provided by C-Span, Jeffrey Thompson speaks at a ceremony to remember Dorothy Height. U.S. Attorney Ronald C. Machen Jr. said that Jeffrey E. Thompson’s accounting firm was an “assembly line” for illegal political donations. (Uncredited/AP)

The corruption investigation that has roiled D.C. politics for two years has uncovered a highly organized operation inside an accounting firm that funneled illegal contributions to scores of political campaigns, according to new court documents filed Thursday.

For nearly a decade, Jeffrey E. Thompson’s accounting firm was an “assembly line” for donations, U.S. Attorney Ronald C. Machen Jr. said. The documents suggest that many employees or their relatives wrote checks to campaigns for D.C. Council, mayor, Congress and other offices to disguise the fact that the money was actually coming from Thompson.

Thompson then reimbursed employees for their “straw” donations through his firm’s accounting and payroll operations, according to the documents.

The allegations that Thompson orchestrated such a far-reaching scheme follow accusations aired last year that he was the secret financier of a $635,000 effort to help the 2010 mayoral campaign of Vincent C. Gray. Those accusations of a “shadow” campaign have dogged Gray (D) since he took office. The mayor has not been charged and denies any wrongdoing.

Prosecutors have said the investigation will move quickly in the coming weeks. They appear to be methodically building a case against Thompson, who was for years one of the District’s largest contractors and who, court records show, is the subject of a grand jury investigation.

D.C. corruption scandals: A look at the investigations and where they stand.

The documents filed Thursday show that Thompson’s allegedly illegal campaign contributions went beyond the shadow mayoral campaign to include years of disguised donations to high-profile politicians other than Gray, including Del. Eleanor Holmes Norton (D-D.C.) and then-Sen. Arlen Specter (D-Pa.).

Thompson’s attorney, Brendan V. Sullivan Jr., declined to comment on the case.

Thursday’s revelations came at the plea hearing of Lee A. Calhoun, an employee of Thompson’s accounting firm, then called Thompson, Cobb, Bazilio and Associates. Thompson has since left the firm. Calhoun admitted in federal court that he contributed $160,000 in his name and the names of relatives to disguise that Thompson was the real donor.

The charges filed against Calhoun last week represented the first time that prosecutors implicated someone who worked directly for Thompson. Thompson is identified in court papers as “Executive A,” but Calhoun’s attorney identified Thompson and his firm in an interview with reporters after Thursday’s hearing.

A second associate of Thompson’s, Stanley L. Straughter, was charged Thursday with a similar campaign violation. Straughter’s attorney, Steven J. McCool, declined to comment on the charge. But Calhoun’s attorney, Edward B. MacMahon Jr., said after Calhoun’s hearing that several people close to Thompson will be charged with similar offenses.

“The people that worked there and the people that were friends of Mr. Thompson were told to do these things, were convinced to do these things, and that’s why they did them,” MacMahon said. He said “many” other employees of the firm were involved, but he declined to speculate on how much money was ultimately donated. “It’s a big number,” he said.

The court documents filed Thursday described an elaborate operation inside the accounting department of Thompson’s firm to keep track of covert donations. Thompson allegedly directed employees and their relatives to become “straw donors” for numerous fundraising events primarily held at the firm’s offices on 15th Street NW.

Reimbursement checks for political contributions were paid through the firm’s accounting department and labeled as “advances” on bonuses, according to the plea deal signed by Calhoun. They often contained an added sweetener of $100 or so, Calhoun said in court.

In Calhoun’s case, the firm allegedly maintained an account in which payments, including the reimbursements, were recorded. The firm also took steps to deal with the tax implications of the alleged straw donations, according to the plea, overstating Calhoun’s income to offset the tax consequences of the reimbursement checks.

Calhoun, 65, told U.S. District Judge Colleen Kollar-Kotelly that his former boss pressed him to make “larger and larger contributions.”

Checks in Calhoun’s name went to numerous candidates, records indicate, not only Norton, Gray and Specter but also former mayor Adrian M. Fenty (D), five sitting D.C. Council members and Maryland Gov. Martin O’Malley (D).

“He offered many assurances that it was legal,” said Calhoun, who added that he quickly realized that hiding the true source of the contributions was illegal.

Calhoun, who lives in Silver Spring, faces a maximum sentence of one year in prison and a fine of up to $50,000.

Federal campaign finance records list at least 35 donors besides Thompson and Calhoun who listed Thompson, Cobb, Bazilio and Associates as their employer. They donated a combined $338,000 to presidential and congressional campaigns and various political action committees, most often giving the maximum amount legally allowed.

Straughter and a relative are listed in federal campaign records as donating $61,800 to federal campaign committees since 2002, often donating to the same candidates and committees supported by Thompson.

The charging document in Straughter’s case alleges that he gave illegal donations of corporate money to congressional campaigns in 2010. Records show that Straughter, who was a contractor for Thompson’s firm, made two donations that year: $2,300 to Norton and $2,000 to Specter.

A Norton campaign spokeswoman did not return messages seeking comment. Specter, who was defeated in the 2010 Democratic primary, died in October.

Calhoun remains an employee of Bazilio Cobb Associates, the successor to Thompson’s firm. That firm issued a statement Friday acknowledging that “employees and their family members and friends were ensnared by the aggressive solicitation of campaign contributions” under Thompson’s ownership, which ended in July.

“The current leadership has and will take responsibility for any actions or inactions that wittingly or unwittingly facilitated this conduct,” the statement said.

The firm said it was “relieved that the cloud it has been under for the past 15 months will lift soon.”