(CNN) -- The accused terrorist who said he was tortured into making a false connection between al Qaeda and Iraq has died in a Libyan prison, human rights monitors said Tuesday.

Colin Powell featured Ibn al-Shaykh al-Libi's claim in a presentation to the U.N. Security Council.

Ibn al-Shaykh al-Libi's allegation that Iraqi agents trained al Qaeda operatives in the use of chemical and biological weapons was "pivotal" to the Bush administration's case for the 2003 invasion of Iraq, said Stacy Sullivan, a counterterrorism adviser for the U.S.-based Human Rights Watch.

"He's a fairly significant figure in the counterterrorism world, and his testimony I would say provided the linchpin for the invasion of Iraq," she said.

Then-Secretary of State Colin Powell featured al-Libi's claim in the dramatic presentation he made to the U.N. Security Council just weeks before the invasion, citing it as evidence of ties between Iraqi dictator Saddam Hussein's government and the terrorist network responsible for the 2001 attacks on New York and Washington. Al-Libi recanted his account after the invasion, and no other evidence supported the story, the Senate Intelligence Committee reported in 2006.

Al-Libi's death was first reported by a Libyan newspaper over the weekend and confirmed by Human Rights Watch researchers Monday, Sullivan said. The newspaper report said he had committed suicide, and the group is asking Libya to conduct a "full and transparent" investigation into his death.

Researchers from the New York-based organization were "stunned" to discover him in a Libyan prison in April during a fact-finding mission to the North African country, Sullivan said. The United States did not transfer al-Libi to the Guantanamo Bay prison camp with other top al Qaeda captives in 2006, and Washington had never confirmed what happened to him, she said.

When prison officials pointed out al-Libi to the researchers, he refused to talk to the group. He told them, "Where were you when I was being tortured in American jails?" Sullivan said.

"He got really angry and walked away," she said.

The Libyan newspaper Oea reported Sunday that al-Libi, born Ali Mohammed Abdelaziz al-Fakhry, had committed suicide in a prison cell in his native country. He had been getting regular visits from his family, the last one in late March, the newspaper reported.

"Upon discovering his body, the police and a doctor were dispatched to the prison immediately to start the investigation," Oea reported.

In Washington, U.S. State Department spokesman Ian Kelly said the United States was investigating the report, but had no immediate comment.

"I have to refer you to the government of Libya for any details regarding the matter," he said.

Al-Libi had trained al Qaeda operatives in Afghanistan and was captured in Pakistan in late 2001, according to the Intelligence Committee report. After the invasion of Iraq, he said his statements connecting Iraq to al Qaeda had been made in response to threats by American interrogators and by beatings at the hands of intelligence agents in Egypt, where he was transferred in 2002.

"I think he would be able to tell us more about the secret CIA prison program and the rendition program," Sullivan said.

Al-Libi told the CIA in 2004 that he had made up the story "in order to gain better treatment" and avoid being shipped to Egypt, the Intelligence Committee reported. Egypt denies wrongdoing, but was widely believed to have tortured prisoners in its custody, according to an April report from the Senate Armed Services Committee.

As early as February 2002, the U.S. Defense Intelligence Agency warned that al-Libi "was likely intentionally misleading his debriefers," the Intelligence Committee report states. The CIA supported al-Libi's account, but added language to its January 2003 assessment noting that he would not have had first-hand knowledge of any weapons training, according to the committee.

The Bush administration argued that the invasion of Iraq was necessary because Iraq was concealing chemical, biological and nuclear weapons programs from international inspectors and could have shared those weapons with terrorists. No such stockpiles were found after the invasion, and the independent commission that investigated the 2001 attacks found no evidence of a collaborative relationship between the two entities.

CNN's Amir Ahmed contributed to this report.

All About Al Qaeda • Iraq • Torture