Khalid al-Masri was "rendered" – a term for extrajudicial transfers of prisoners – to the CIA in January 2004 after being arrested by Macedonian border authorities, who mistook him for an al-Qaeda suspect.
While in CIA custody he was severely beaten, stripped, shackled and sodomized with a suppository as part of a process the agency called "capture shock".
He was later flown to a CIA site in Kabul, Afghanistan, where he was beaten, kicked and subjected to other abuse in a prison called the "Salt Pit".
Officers believed that he had "key information that could assist in the capture of other al-Qaeda operatives that pose a serious threat of violence or death to US persons and interests", the report said.
The decision to hold al-Masri was already problematic at that point, as standards established by the US government for detentions required that the person detained poses a serious threat.
The Senate report cites a 2007 review of the case by the CIA Inspector-General, which found that "available intelligence information did not proide a sufficient basis to render and detain Khalid al-Masri".
But even after interrogators had determined that al-Masri had been confused with a suspect who had a similar name, posed no threat and had no information, their superiors couldn't decide on whether or not to release him.
Finally, in May, al-Masri was flown to Albania, dumped by the side of the road and given €14,500 to buy his silence.
He had never been charged or had his case heard by a judge, and the German government was unaware of his detention.
Battle for justice
Chancellor Angela Merkel told journalists in 2005 that the United States government admitted making a mistake in al-Masri's case, but the US has fought a bitter legal battle to prevent the truth emerging ever since.
A lawsuit filed in the US was dismissed on "state secrets" grounds, while the Macedonian government has refused to admit its involvement.
And the German government has failed to disclose what it knew about the case or take any legal action.
But the European Court of Human Rights found in December 2013 that Macedonia had violated al-Masri's rights, in the first court decision to describe CIA "enhanced interrogation" techniques as torture.
It ordered the Macedonian government to pay him €60,000.
Al-Masri was sentenced by a court in Kempten to seven months in prison in December 2013 after assaulting a prison guard during a previous sentence. He has been in and out of prison and the courts several times since being released by the CIA.
As for the officers responsible for his treatment, the CIA Director decided that they shouldn't be punished.
The Senate report shows that "the Director believes the scale tips decisively in favour of accepting mistakes that over-connect the dots against those that under-connect them".
In its report on its own failings in the al-Masri case to the US Congress, the CIA defended the actions of its Counter-terrorism Centre (CTC) in the case.
Officers there were in a "high threat environment… essentially identical to the one in which CTC employees… previously had been sharply criticized for not connecting the dots prior to 9/11", they said.
US President Barack Obama said on Tuesday that "when we make mistakes, we admit them" of the CIA's use of torture, promising to stop such methods being used in future.
But the NGO Human Rights Watch said that the CIA officials and members of the government who approved and oversaw the torture programme should be brought before the courts.
As well as the abuse directed at al-Masri, dozens of other prisoners were subjected to brutal beatings, kept awake for up to 180 days on end and almost drowned during "waterboarding".
Apart from al-Masri, 25 other innocent people were wrongfully held and subjected to abuse.
Amnesty International held a press conference in Berlin on Tuesday to mark World Human Rights Day, which fell on the same day as the report's release.
International law expert Maria Scharlau told reporters that "abuses often happen in the name of national security. The justification of torture by the USA in the 'War on Terror' was a negative signal" to other governments.
Amnesty called on the international community to act decisively against torture and respect the United Nations Anti-Torture Convention of 10 December 1984.
US Senate Intelligence Committee: Committee Study of the Central Intelligence Agency's Detention and Interrogation Program