WASHINGTON (AP) — Federal prosecutors made an unannounced visit this week to a construction site at Federal Reserve headquarters that is the focus of an investigation into a $2.5 billion renovation project, according to two people familiar with the visit.

Two prosecutors and an investigator from U.S. Attorney Jeanine Pirro’s office were turned away on Tuesday by a building contractor and referred to Fed attorneys, one of the people said. The two people familiar with the visit spoke on condition of anonymity because they weren’t authorized to publicly discuss an ongoing investigation.

The visit underscores that the Trump administration is not backing down from its investigation of the Fed and its chair, Jerome Powell, even though the probe has delayed the confirmation of a new chair nominated by President Donald Trump. The investigation is focused on cost overruns and brief testimony about the project last summer by Powell. Trump confirmed in an interview that aired Wednesday on Fox Business that he wants to continue the probe.

Last month, during a closed-door hearing before a federal judge, a top deputy from Pirro’s office conceded that they hadn’t found any evidence of a crime in their investigation of the headquarters project.

Robert Hur, an attorney for the Federal Reserve board of governors, sent an email to Pirro’s prosecutors about their visit and their request for a “tour” to “check on progress” at the construction site. Hur’s email, which The Associated Press has viewed, noted that U.S. District Judge James Boasberg concluded that their interest in the Federal Reserve’s renovation project was “pretextual.”

“Should you wish to challenge that finding, the courts provide an avenue for you; it is not appropriate for you to try to circumvent it,” Hur wrote.

Sen. Thom Tillis, a North Carolina Republican who is a key member of the Senate Banking Committee, has vowed to vote against Kevin Warsh, Trump’s nominee to replace Powell as Fed chair, until the investigation is dropped. With the committee closely divided on partisan lines, Tillis’ opposition is enough to block Warsh from receiving the committee’s approval.

Tillis on Wednesday criticized the investigation as “bogus, ill-timed, ill-informed” and repeated that seven Republican members of the banking panel have said they do not believe Powell committed a crime when he testified last June.

Tillis also said there aren’t enough votes on the committee or in the broader Senate to do an end-run around the committee and get Warsh confirmed some other way.

“There really is no path,” he told reporters, adding that Pirro and her aides were “asleep at the switch” because the investigation has essentially delayed Powell’s departure from the Fed, despite Trump’s obsessive criticism of the Fed chair. Powell has now said he won’t leave until the investigation is resolved.