The F.B.I. has opened a criminal investigation into accusations that Department of Veterans Affairs officials manipulated medical waiting lists and delayed care for thousands of veterans, the F.B.I. director, James B. Comey, said Wednesday. Testifying before the House Judiciary Committee, Mr. Comey said F.B.I. agents in Phoenix were leading the investigation. The veterans medical center in Phoenix is at the heart of the national scandal, which led to Eric Shinseki's resignation as Veterans Affairs secretary last month. The inspector general of the Department of Veterans Affairs had already said that the Justice Department was looking into the matter, but Mr. Comey's testimony was the first indication that a criminal investigation was underway.