Updated — Nov. 17, 4:15 p.m. ET
The parents of slain aid worker Peter Kassig said Monday that their son believed in good.
A prosecutor in Paris identified a French 22-year-old convert to radical Islam in the video showing Kassig's beheading, the Associated Press reported.
Francois Molins identified the man as Maxime Hauchard, and said that he has been on the radar of French authorities since he left for Syria in 2013 under cover of humanitarian action.
Molins also said Monday that another Frenchman could be among the fighters in the video, but it is too early to determine for certain.
In a new video posted online Sunday, ISIS executed Abdul-Rahman Kassig, a former U.S. Army Ranger who was captured Oct. 1, 2013.
Kassig was a humanitarian aid worker who converted to Islam while in captivity. He also changed his name from Peter to Abdul-Rahman. He was en route to Deir Ezzour, in Syria, while working for his aid organzation when he was first "detained."
The video shows a black-clad man with a British accent standing over a severed head.
"This is Peter Edward Kassig, a U.S. citizen of your country," the man says to the camera. "Peter, who fought against the Muslims in Iraq while serving as a soldier under the American army doesn't have much to say."
The White House told the AP on Sunday afternoon that they reviewed the Islamic State video and can confirm the death of Kassig.
"Today we offer our prayers and condolences to the parents and family of Abdul-Rahman Kassig, also known to us as Peter," President Barack Obama said in a statement. "We cannot begin to imagine their anguish at this painful time."
"While ISIL revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction, Abdul-Rahman was a humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict," the president said, using an alternative acronym for the terrorist group.
"Today we grieve together, yet we also recall that the indomitable spirit of goodness and perseverance that burned so brightly in Abdul-Rahman Kassig, and which binds humanity together, ultimately is the light that will prevail over the darkness of ISIL," the president said.
Kassig's parents, Ed and Paula, released a statement Sunday morning saying they were waiting for the government to confirm the authenticity of the video, and had "no other statement at this time."
"We ask that you please respect our privacy," the Kassigs said.
ISIS first publicly threatened to kill Kassig in early October. In the ensuing weeks, his family appealed directly to ISIS and the organization's self-described caliph, Abu Bakr al-Baghdadi, for mercy. In their appeals, Kassig's family cited his humanitarian work and conversion to Islam.
In late October, a prominent Syrian jihadi also asked ISIS to spare Kassig's life. Abu Omar Aqidi is a member of a group linked to al-Qaeda, but said he was treated once by Kassig, who was trained as an EMT and medic.
A number of Kassig's friends also called for his release.
Before becoming an aid worker, Kassig was an Army Ranger. He was deployed to Iraq in 2007, and later honorably discharged for medical reasons.
The new video is both longer and far more gruesome than others released by the terror organization.
Running nearly 16 minutes, the video begins with a news-like presentation, then eventually shows the mass execution of men described as soldiers of Syrian President Bashar al-Assad.
Kassig is the fifth Westerner executed by ISIS. However, unlike previous execution videos, the one released Saturday does not conclude with another threat against a specific western hostage. The terror organization has previously released videos showing the executions of British citizens Alan Henning and David Haines, as well as American journalists James Foley and Steven Sotloff.
UPDATE: Defense Secretary Chuck Hagel issued a statement Sunday saying Kassig's killing was "one more reminder of the Islamic State of Iraq and the Levant's (ISIL) ruthless barbarity":
On behalf of all the men and women of the Department of Defense, I extend my deepest condolences to the family of Abdul-Rahman Kassig, also known to us as Peter, who time and again volunteered his service during times of war -- first as an Army Ranger in Iraq, and later as a devoted humanitarian, providing aid to victims of the conflict in Syria. Like his fellow veterans of the 9/11 generation, his strong desire to continue making a difference in the world after serving in uniform -- to continue leading a life of purpose -- is an inspiration to us all. His brutal murder is one more reminder of the Islamic State of Iraq and the Levant's (ISIL) ruthless barbarity; there is no starker contrast between the inhumanity of ISIL and the bright and generous spirit of the young man they murdered. As we join his loved ones in mourning his loss, we also celebrate his service, and we celebrate his commitment -- a lifetime commitment to, as he said, "stand beside those who might need a helping hand.