WASHINGTON – Federal investigators are probing Hillary Rodham Clinton’s 2008 presidential campaign as part of an effort to expand their case against a District of Columbia businessman alleged to have financed below-the-radar political operations, people familiar with the investigation said.

The businessman, Jeffrey Thompson, allegedly provided more than $600,000 to fund secret get-out-the-vote efforts to help Clinton in at least four primary states. Prosecutors do not expect to pursue a separate criminal case against Clinton’s campaign, these people said.

In a wide-ranging effort to expand their 18-month-old corruption case against Thompson, federal investigators have reviewed hundreds of emails and interviewed more than a dozen Clinton campaign staffers and supporters.

Investigators have focused their questioning on the role of Minyon Moore, a senior campaign adviser who allegedly helped arrange a “street teams” operation in Texas, Indiana, North Carolina and Pennsylvania that was financed by Thompson and run by marketing executive Troy White, according to those several sources, who spoke on the condition of anonymity because the probe is continuing.

Moore says she was unaware of any inappropriate activities and is cooperating with authorities.

In a series of interviews over several months, investigators have asked former Clinton campaign officials whether they knew White or were aware of his election-related efforts, which were not publicly disclosed as required by law, some of those familiar with the probe said.

These people said the questioning also focused on emails between Moore and campaign staffers about voter turnout strategies in majority-African-American and Hispanic urban precincts, as well as about the campaign’s schedule of public events, signs and literature. In many instances, these people said, Moore sought information from staffers who did not know she was also allegedly helping White’s “street teams” operation.

“Minyon, she was influential in the campaign,” said one former campaign official.