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.