Federal prosecutors are investigating Federal Reserve Chair Jerome Powell over a $2.5 billion renovation of the Fed’s Washington, D.C., headquarters and his related congressional testimony.

Powell said the probe reflects longstanding frustration from President Trump over the Fed’s refusal to cut interest rates as quickly as he demanded, warning it threatens the Fed’s independence.

The investigation involves grand jury subpoenas and is being overseen by the U.S. Attorney’s Office for D.C., led by Trump appointee Jeanine Pirro.

Lawmakers and analysts have criticized the probe as politically motivated, warning it could undermine confidence in the central bank and U.S. markets, CNBC has reported.

Trump has repeatedly targeted Powell over interest rates and the renovation costs, signaling intentions to replace him once his term ends in May.