The department sent a copy of the Teapot Dome lease granting drilling rights to Harry Sinclair's Mammoth Oil Company, along with a statement that there had been no competitive bidding because military preparedness was involved—Mammoth was to build oil tanks for the Navy as part of the deal. This satisfied some people, but some conservationists, such as Gifford Pinchot, Harry A. Slattery, and others, pushed for a full investigation into Fall and his activities. They got Wisconsin Senator Robert M. La Follette to begin a Senate investigation into the oil leases. La Follette persuaded Democratic Montana Senator Thomas J. Walsh to lead the investigation, and Walsh read through the truckload of material provided by the Interior Department through 1922 into 1923. The documents included a letter from Harding stating that the transfer and leases had been with his knowledge and approval.[213]
Hearings into Teapot Dome began in October 1923, two months after Harding's death. Fall had left office earlier that year, and he denied receiving any money from Sinclair or Doheny; Sinclair agreed. The following month, Walsh learned that Fall had spent lavishly on expanding and improving his New Mexico ranch. 