Attorneys for Robert F. McDonnell want to subpoena the U.S. Food and Drug Administration and the Securities and Exchange Commission for documents they say might help them impeach a key government witness in the corruption case against the former Virginia governor.

In filings late Thursday, the attorneys asked the court to issue subpoenas that would provide them with investigative files and other material from the FDA’s interaction with Richmond businessman Jonnie R. Williams Sr. and Star Scientific, the struggling dietary supplement company he used to run.

They asked for subpoenas for similar information from the SEC about Williams and several companies with which he has been affiliated.

McDonnell and his wife, Maureen, are charged with repeatedly asking Williams for loans and gifts of money, clothes, trips and private plane rides. In exchange, prosecutors allege the couple lent Williams and his company the prestige of the governor’s office by, among other things, promoting Star’s dietary supplement and arranging meetings between Williams and senior state health officials.

The McDonnells have pleaded not guilty, and a trial is scheduled for July 28.

In previous filings, the McDonnells’ attorneys have attacked Williams’s credibility. They wrote Thursday that information from the SEC and the FDA might provide key context to his testimony, noting that Star has been the target of SEC and FDA probes.

“One of the key questions regarding Mr. Williams’s testimony will be what potential charges he and his company are facing as a result of any federal investigations, and whether the Government has offered Mr. Williams immunity in connection with such investigations in exchange for testimony against the McDonnells,” the attorneys wrote.

“For that reason, the investigative files are essential to understanding Mr. Williams’s testimony. Star Scientific’s business and marketing plans, implicated in both the securities and marketing investigations, are also deeply intertwined with the Government’s substantive allegations in this case.”

The FDA in December issued a regulatory warning to Star, alleging the company had been improperly selling the dietary supplement backed by the governor. Star also acknowledged to its shareholders it was the subject of a securities investigation but said last year it did not expect to face charges.

McDonnell’s defense attorneys wrote that they had been unsuccessful in trying to get full investigative files from both agencies through Freedom of Information Act requests, and prosecutors had also refused to turn them over voluntarily.

As of late Thursday, prosecutors had not responded to the filings. But in a March 6 letter attached to defense attorneys’ filing, they wrote that they did not consider the FDA and SEC investigations parallel to their own, and they had already turned over the documents in their possession. They wrote that they had been told the SEC’s informal inquiry was closed, and the governor and his wife were the only designated targets of their public corruption investigation.