Until this week, the case against prominent D.C. businessman Jeffrey E. Thompson had largely focused on his alleged secret spending on behalf of Mayor Vincent C. Gray and other local politicians.

Over the last two years, prosecutors have been deliberately building a case against Thompson, who is the former owner of accounting and health-care firms that did billions of dollars in city business.

The revelations on Wednesday that Thompson allegedly spent more than $608,000 to help Hillary Clinton’s 2008 presidential campaign emerged during a court hearing for a New York marketing executive. Troy White, the owner of Wytehouse Marketing Inc., admitted in court papers that he was paid hundreds of thousands of dollars by Thompson to organize “street teams” to distribute campaign materials for Clinton during her primary battle with then-Illinois senator Barack Obama.

A search of federal campaign records found no evidence that Thompson had reported the alleged expenditures to the Federal Election Commission, as required by campaign finance laws.

White, a Howard University alumni, was just the latest person in Thompson’s vast network to be charged in the far-reaching federal investigation. Prosecutor’s remain focused on the D.C. businessman and are unlikely to open a separate probe into Clinton’s unsuccessful campaign, according to people familiar with the case who spoke on condition of anonymity because the investigation is ongoing.

Through a series of plea agreements, prosecutors are establishing a pattern of how Thompson allegedly worked around campaign finance limits and disclosure laws to help candidates he did not want to support publicly.

Thompson has not commented publicly since federal investigators raided his home and offices in March 2012. His attorney Brendan V. Sullivan Jr. declined to comment on this week’s developments.

In June, U.S. Attorney Ronald C. Machen Jr. told reporters said that the investigation was moving “extremely fast in pace.” The timeline changed in July, however, after prosecutors and Thompson agreed behind the scenes to waive the statute of limitations for the government to file certain charges.

The extension of the deadline appeared to open the door for negotiations. But the case has continued to play out in fresh court filings, suggesting that prosecutors are moving ahead.

In August, Gray’s longtime friend and political adviser, Vernon E. Hawkins, pleaded guilty to an obstruction charge. Hawkins admitted drawing up plans and a budget for the $653,000 secret effort that Thompson allegedly funded to benefit the mayor.

Court filings have also outlined how Thompson’s alleged off-the-books spending extended beyond the “shadow campaign.”

Without identifying Thompson by name, Machen has said that Thompson’s former accounting firm was an “assembly line” for donations to local and federal politicians. Thompson’s employees and their relatives wrote checks for candidates, hiding that the money was actually coming from Thompson, according to court documents.

Among the candidates receiving the so-called “straw donations” are Del. Eleanor Holmes Norton (D-D.C.) and the late senator Arlen Specter (D-Pa.).

Neither Thompson nor Gray has been charged, and the mayor has denied any wrongdoing.