ALEXANDRIA, Va. — Two former business associates of Michael T. Flynn, President Trump’s first national security adviser, have been indicted as part of a federal investigation into Turkey’s secret 2016 lobbying campaign to pressure the United States to expel a rival of President Recep Tayyip Erdogan.

Charges against the two former associates, Bijan Kian and Ekim Alptekin, were unsealed on Monday in an Alexandria, Va., courtroom. The two men were indicted last week as part of a conspiracy to violate federal lobbying rules, and Mr. Alptekin was also charged with making false statements to F.B.I. investigators.

The indictment is further evidence of a broad crackdown on unregistered foreign lobbying growing from the inquiry by Robert S. Mueller III, the special counsel who has investigated foreign flows of money from Ukraine, Turkey and other countries devised to manipulate decision-making in Washington. Mr. Mueller referred the Turkey case back to prosecutors in Northern Virginia.

The indictment said that the two men sought to conceal that Turkey was directing the work, and that high-level Turkish officials approved the budget for the project and were given regular updates by Mr. Alptekin about the campaign’s progress. Mr. Flynn’s firm — Flynn Intel Group — received a total of $530,000 for its work.