Moroccan-born Mohammed al Alami, who was released in 2006, is the first former Guantanamo detainee to die in battle in the Syrian civil war, analysts say.
The video, first reported by The Miami Herald, was posted by Harakat Sham al-Islam, one of the Islamist brigades fighting against Syrian President Bashar Assad. It showed the Aug. 5 funeral in which Alami is praised by a rebel leader for enduring "the prison of the Americans in Guantanamo for five years ... where he did not reform or change."
Defense Department officials had no comment.
Aaron Zelin, a fellow at the Washington Institute for Near East Policy who monitors jihadi activities at the website Jihadology.net, said the video appeared to confirm rumors that had been circulating for several weeks about a former Guantanamo prisoner being killed in Syria.
"The sources seem to be legitimate," he said, adding that the video was posted on a known jihadi website.
Alami fought in Afghanistan and was captured in Pakistan after the September 11 attacks.
He was sent to the U.S. detention facility in Cuba on Feb. 2, 2002 "because of his knowledge of Taliban recruitment, training and tactics as well as his possible affiliation with al Qaeda," according to Pentagon records made public by the anti-secrecy group WikiLeaks.
He was repatriated to Morocco on Feb. 7, 2006, the records show.
Alami's death was likely to feed concerns about the risks of releasing detainees from Guantanamo, even though many have not been formerly charged.
"It has implications for the debate about what the United States does with the individuals still in Guantanamo and what might happen if they return home, or are released," said Zelin.
There are a number of rebel brigades made up entirely of non-Syrians, the United Nations says, underlining how the 2-1/2-year-old conflict has pulled in neighboring countries and widened sectarian fault lines across the region.
The growing involvement of foreign, Islamist fighters has added to Western reluctance to step in or arm the rebels. Reuters correspondents have met British, Libya and Tunisian militants in Syria who say they fight against Assad.
Lebanon's Shiite Muslim Hezbollah militia has been fighting alongside Syrian government forces.
"Former Guantanamo detainees are held in very high light," said Zelin. "They were imprisoned by Americans and they still kept faith in God and the cause of Jihad. They are viewed as heroes."
Last month the U.S. repatriated two Guantanamo detainees to Algeria as part of its ongoing effort to close the prison.
So far, the U.S. has released 606 detainees. There are currently 164 prisoners at Guantanamo, including 84 cleared for release years ago.
Obama promised to do away with the facility during his 2008 presidential campaign, citing its damage to the U.S. reputation around the world, but Congress put tough restrictions on detainee transfers in January 2011.
A second Moroccan ex-Guantanamo detainee, Ibrahim bin Shakaran, the leader of Harakat Sham al-Islam, is seen in the video giving the funeral eulogy for Alami. He was described as an al Qaeda recruiter in Iraq, according to a 2008 Defense Intelligence Agency list of 37 Guantanamo detainees "confirmed or suspected" of having returned to terrorism.
The number of former Guantanamo recidivists was estimated by the DIA in 2008 to be "about 7 percent of those transferred from U.S. custody."
Alami, was the second known former Guantanamo detainee to be killed this year, Zelin added. A Saudi second-in-command of al Qaeda in the Arabian Peninsula (AQAP) was killed in a U.S. drone attack in Yemen in July.
Said al-Shehri was described by U.S. officials as one of the most important al Qaeda-linked militants to be released from Guantanamo, where he was taken in January 2002 after Pakistan handed him to U.S. authorities.