Casey was due to appear before the HSCI on 16th December. The day before, CIA physician, Dr. Arvel Tharp went to visit Casey in his office. According to Tharp, while he was being examined, Casey suffered a seizure. He was taken to Georgetown University Hospital and was not able to appear before the HSCI. Tharp told Casey he had a brain tumor and that he would have to endure an operation. Casey was not keen and asked if he could have radio therapy instead. However, Tharp was insistent that he needed surgery.
Casey entered the operating room on 18th December. The tumor was removed but during the operation, brain cells were damaged and Casey lost his ability to speak. As his biographer, Joseph E. Persico, points out (The Lives and Secrets of William J. Casey): "one school of rumors ran, the CIA or the NSC or the White House had arranged to have a piece of the brain removed from the man who knew the secrets".
Robert Gates now became acting director of the CIA. He claimed that he was not involved in the Iran-Contra operation. As Lawrence E. Walsh pointed out in Iran-Contra: The Final Report "Gates consistently testified that he first heard on October 1, 1986, from the national intelligence officer who was closest to the Iran initiative, Charles E. Allen, that proceeds from the Iran arms sales may have been diverted to support the contras. Other evidence proves, however, that Gates received a report on the diversion during the summer of 1986 from DDI Richard Kerr. The issue was whether Independent Counsel could prove beyond a reasonable doubt that Gates was deliberately not telling the truth when he later claimed not to have remembered any reference to the diversion before meeting with Allen in October."William Joseph Casey died on 6th May, 1987. After his death, Oliver North testified that Casey had played an important role in the Irangate Scandal. Lawrence E. Walsh argued in Iran-Contra: The Final Report: "There is evidence that Casey played a role as a Cabinet-level advocate both in setting up the covert network to resupply the contras during the Boland funding cut-off, and in promoting the secret arms sales to Iran in 1985 and 1986."