The Senate Intelligence Committee is expected to release its long-awaited report on Tuesday, detailing the controversial interrogation methods used by the Central Intelligence Agency on al-Qaeda suspects after the 9/11 attacks.

US embassies worldwide are on heightened security alert ahead of the landmark torture report, which is the third of its kind in 15 years, after the official commissions into the 9/11 plot and Saddam Hussein's obsolete illicit weapons programmes.

"The administration has taken the prudent steps to ensure that the proper security precautions are in place at US facilities around the globe," a White House spokesman said, adding the report posed a "greater risk" to US facilities.

Who wrote the report and what will it entail?

The report is 6,000 pages long and was written by Democratic staff members of the Senate Intelligence Committee, between 2009 and 2013. Republican committee members chose not to participate in the inquiry. Today, only the 480-page executive summary of the $40 million report will be released.

The document is a review of more than five million highly classified CIA documents and is expected to contain graphic details of the agency's use of torture on al-Qaeda suspects after the September 11 attacks.

It is expected to contain details of techniques such as being kept awake and waterboarding, used against suspects held in an international network of prisons dubbed "black sites", the countries which hosted the jails and the names of CIA personnel. The report is expected to say harsh interrogation, including sexual threats being made with a broomstick and mock execution threats, failed to deliver appropriate results.

President Barack Obama ceased the CIA interrogation programme, "Rendition, Detention and Interrogation", when he took office in 2009.

What do we already know about the report?

Through Freedom of Information requests, leaks and the official release of information, the public are aware of some details from the report. Obama has already described the CIA's techniques as "torture" in a press conference earlier this year and the public are aware of the list of interrogation techniques used. Details of waterboarding, stress positions and hypothermia have all been released.

As reported by Reuters, the report "describes how senior al-Qaeda operative Abdel Rahman al Nashiri, suspected mastermind of the 2000 bombing of the USS Cole, was threatened by his interrogators with a buzzing power drill".

Information about prisons in Poland and Romania has already been released, as have the names of around 100 prisoners who the CIA said had been in the secret prisons.

Why was the report delayed?

Publication of the document has been held amid disagreements in Washington over what should be made public. The report was approved by the Senate Intelligence Committee in December 2012, but the CIA stated it contained inaccuracies and penned a secret rebuttal. The full report remains classified.

The Senate committee last year accused the CIA of spying on its work on the report by removing staff members' documents from computers, but the agency stated the committee was examining material it had no authorisation to look at. The CIA director, John Brennan, eventually admitted the agency had spied.

In August, a heavily amended version of the summary was written by the CIA. However, Democratic Senator Dianne Feinstein claimed too much information was omitted and a compromise has been reached since then.

What will happen next?

The committee's Republicans are expected to release a dissent challenging some of the report's conclusions. The report may resurrect a discussion about accountability, as the State Department allegedly expects questions about whether the government will reopen investigations into the defunct programme.

The Obama administration previously said it investigated those involved in the "enhanced interrogation techniques" and found no reason to bring criminal charges. In preparation for the aftermath of the report, alerts have been issued in Egypt and other embassies worldwide - closing down visa operations and stepping up security.