The FBI is investigating the authority responsible for overseeing the $5.6 billion Dulles Rail project, the latest in a series of revelations that has rocked a board seeking to right itself after scathing reports about ethical lapses in its operations, according to people with knowledge of the investigation.

News of the probe comes just days before the expected arrival of a federal inspector general’s report critical of the Metropolitan Washington Airports Authority’s contracting and hiring practices.

Authority officials remained tight-lipped about the situation.

“To date, the Airports Authority has not been officially notified of any FBI investigation,’’ authority spokesman David Mould said. “The Airports Authority is committed to full cooperation with any government review, as we have been doing with the Department of Transportation Inspector General’s audit.”

But Mould did confirm that subpoenas seeking documents had been served on the authority. Mould referred further inquiries to the FBI, which would neither confirm nor deny an investigation.

The FBI probe, first reported by the Washington Examiner, comes as the authority awaits the final report from the U.S. Transportation Department’s inspector general, which has examined authority contracting and procurement procedures.

At the request of Reps. Frank R. Wolf (R-Va.) and Tom Latham (R-Iowa), the inspector general’s office began looking into the inner workings of the authority, which oversees the $5.6 billion Silver Line rail project and operates Dulles International and Reagan National airports, and the Dulles Toll Road. The authority has 1,400 employees and a nearly $2 billion budget but is not subject to the oversight generally faced by airport authorities in other parts of the country.

The authority’s involvement with the long awaited Silver Line rail project has brought a new level of scrutiny to a board that largely operated out of public view since it was created in 1987.

The interim report, released in May, was critical of the board’s conduct, noting that there were very few controls on how much the board spent on travel and entertainment. It raised questions about the board’s conflict-of-interest and ethics policies. It also cited an incident in which a no-bid contract was awarded to a law firm that employed the wife of a board member.

The report also raised questions about more than $220 million in no-bid contracts. Authority staff members also did not obtain board approval for $3 million in contracts, despite policies that require a board vote.

The board has since changed its travel and entertainment policies and is in the process of revamping its contracting practices.

In July, Transportation Secretary Ray LaHood appointed an accountability officer to oversee efforts to revise the authority’s ethics and contracting policies.