According to the former DCI [Director of Central Intelligence] Chief of Staff, he directed CIA terrorism analysts to prepare objective appraisals of the current terrorist threat, focusing primarily on threats to the U.S. homeland, and to document those appraisals in a memorandum. Initially, the analysts who prepared the threat assessments were not read into the PSP [President's Surveillance Program] and did not know how the threat assessments would be used. ...
After the terrorism analysts completed their portion of the memoranda, the DCI Chief of Staff added a paragraph at the end of the memoranda stating that the individuals and organizations involved in global terrorism (and discussed in the memoranda) possessed the capability and intention to undertake further terrorist attacks within the United States. The DCI Chief of Staff recalled that the paragraph was provided to him by a senior White House official. The paragraph included the DCI's recommendation to the President that he authorize the NSA to conduct surveillance activities under the PSP.
It looks like it was this slipped-in paragraph, inserted by that unnamed "senior White house official," that was used to provide the justification for the Bush administration's warrantless surveillance.
Spencer Ackerman points out that the DCI chief of staff here appears to be John Moseman. It's unclear whether Moseman, if it was he, told the IGs the identity of the White House official.