Planes flying into the World Trade Center. Bodies falling from high-rise windows of the twin towers.
The events of that day - the largest ever attack on American soil - are impossible to forget. The American public responded with fear and anger and the Bush administration responded with a plot for revenge that still haunts the White House and the Pentagon.
Part of that process meant establishing a torture program. Details of the program remain a closely-guarded secret 15 years later.
What we know about the CIA's actions is neatly contained in a 500-page executive summary released in 2014. But there's so much more that we don't know.
America's spy agency has 6700 pages detailing exactly what went on at so-called "black sites" after 2001. At least, they did have 6700 pages.
According to a number of reports, the CIA's internal investigators inexplicably "erased" their only copies of the document.
According to Yahoo News, which first reported the news on Monday, the CIA admitted to accidentally deleting a document and destroying a disk that contained the report.
It adds to speculation of a cover-up and paints a picture of incompetence at the agency's headquarters at Langley, Virginia. A professor of law in New York put it best this week when he said: "It makes you wonder what was going on over there."
Sleep deprivation, waterboarding, nudity, "wallings"
Dianne Feinstein, chairman of the Senate Select Committee on Intelligence, wrote when handing down the executive summary in 2014 that the full report had been provided to a number of different parties.
Those included the White House, the CIA, the Department of Justice, the Department of Defense, the Department of State and the Office of the Director of National Intelligence.
Ms Feinstein admitted the report was "highly critical of the CIA's actions, and rightfully so" but acknowledged that it was important to remember the fear that existed in the months and years after 2001.
"It is worth remembering the pervasive fear in late 2001 and how immediate the threat felt," she wrote.
"Just a week after the September 11 attacks, powdered anthrax was sent to various news organisations and to two US Senators. The American public was shocked by news of new terrorist plots and elevations of the colour-coded threat level.
"We expected further attacks against the nation."
It's not yet clear if the staffer responsible for deleting the full report into the agency's Detention and Interrogation Program will be punished.
Between 2001 and 2009, the CIA pushed ahead with a "brutal" interrogation program that involved indefinite secret detention and torture. That was, of course, a violation of US law.
Ms Feinstein acknowledged that the full report "includes comprehensive and excruciating detail" of interrogation techniques used on 119 individuals held during that period.
Among the executive summary's findings were that interrogators used "wallings" (slamming detainees against walls), sleep deprivation, nudity, waterboarding and rectal feeding.
"The waterboarding technique was physically harmful, inducing convulsions and vomiting," the summary found.
"Internal CIA records describe the waterboarding of (one detainee) as evolving into a 'series of near drownings'."
Other detainees were given ice water baths and placed inside a coffin-shaped box. The impacts were long-lasting.
"Throughout the program, multiple CIA detainees who were subjected to the CIA's enhanced interrogation techniques and extended isolation exhibited psychological and behavioural issues, including hallucinations, paranoia, insomnia, and attempts at self-harm and self-mutilation," the report read.
"Multiple psychologists identified the lack of human contact experienced by detainees as a cause of psychiatric problems."
How a 6700-page report goes missing
The CIA's internal watchdog was forced to admit the embarrassing mistake when pressed by reporters.
Yahoo reported deleting the document was "an inadvertent foul-up" that was privately disclosed to the Senate Intelligence Committee last summer but has never been made public. It comes at a time when debate about the report's release is more fervent than ever.
On Friday, a federal appeals court rejected efforts to see the full version of the report made public. The American Civil Liberties Union is suing to obtain the full version but it's not yet clear if those efforts will deliver a result.
Douglas Cox, a University of New York School of Law professor said he was shocked at the news the full report had been lost.
"It's breathtaking that this could have happened, especially in the inspector general's office - they're the ones that are supposed to be providing accountability within the agency itself," he said.
Contained within the report are other "breathtaking" details, including how CIA personnel were never held accountable for violating CIA policies.
"Significant events, (including) the death and injury of CIA detainees, the detention of individuals who did not meet the legal standard to be held, the use of unauthorised interrogation techniques against CIA detainees and the provision of inaccurate information on the CIA programs did not result in appropriate, effective, or in many cases, any corrective actions," the report noted.