Kurnaz, a Turk who was born and lives in Germany, was arrested during a trip to Pakistan in autumn 2001 and delivered to US authorities in exchange for a payment of $3,000.
Kurnaz spent several nightmarish weeks at the US base in Kandahar, Afghanistan before being transferred to the US "war on terror" camp at Guantanamo.
US authorities determined in 2002 that Kurnaz had no terror links, but claimed that he remained a danger because one of his friends had committed a suicide attack - even though the friend in question is alive, and has never been found to have terror ties.
Kurnaz was not released until mid-2006, and only after pressure on Washington from German Chancellor Angela Merkel.
The US House of Representatives Committee on Foreign Affairs invited Kurnaz to testify via video conference as part of a hearing on Guantanamo detainees who are being considered for release but cannot find a host country to take them. Taking into account the difference in time between the United States and Europe, it was already late in Germany by the time committee speaker Democrat Bill Delahunt gave Kurnaz an opportunity to speak.
Kurnaz, his hair trimmed short and dressed in a black suit, gripped his notes and began to speak, but a technical difficulty prevented the audio from reaching Washington.
Some reporters and members of the public gave up waiting, but after around a half hour Kurnaz was able to be heard - and he recounted some of the horrific details of his travail.
"I did nothing wrong and I was treated like a monster," he said, describing acts of torture such as being suspended by his wrists for hours on end, receiving electrical shocks and enduring simulated drowning.
"I know others have died from this kind of treatment," he said. "I suffered from sleep deprivation, solitary confinement, religious and sexual humiliations. I was beaten multiple times," he said. "There was no law in Guantanamo."
Many of the details are included in his book, "Five Years of My Life: An Innocent Man in Guantanamo," recently released in the United States.
Facing the screen, the majority of seats for committee members sat empty. Barely half a dozen lawmakers came to listen to the former detainee, and most were unable to remember his name, with one even calling him "Mr. Karzai."
The first to speak after Kurnaz was finished was ranking member on the committee, Republican Congressman Dana Rohrabacher, who expressed doubts about the testimony and recalled that the United States was "at war" and needed to protect itself even at the price of making some errors.
And when it came to the question of where ex-inmates should go when their home country refuses them, Rohrabacher said: "I would suggest that any of our allies who have criticized the US should take some of them themselves."
It was past 10:00 pm in Germany by the time Democrat Jerrold Nadler spoke. "The people who tortured you were committing crimes under American law," Nadler said. "I hope in the next few years these people will be held accountable.
"I wanted in the name of the United States to express for you my regrets and my apologies," he added, to the sound of timid applause from the public.