Details of the CIA's "enhanced interrogation" techniques, considered by many to be torture, are set to be revealed in a long-awaited report.
The Senate Intelligence Committee's report is expected to find that such interrogations failed to yield life-saving information.
The techniques were used on al-Qaeda terrorism suspects in the years following the 9/11 attacks on the US.
Security was increased at US facilities around the world ahead of publication.
Embassies and other sites were taking precautions amid "some indications" of "greater risk", a White House spokesman said.
The Senate committee's report runs to more than 6,000 pages, drawing on huge quantities of evidence, but it remains classified and only a 480-page summary will be released.