Hundreds mourn beheaded American aid worker Peter Kassig during Muslim funeral service as top Syrian cleric threatens his killers with 'hellfire'

• Ed and Paula Kassig attended a Muslim funeral service held in a mosque in Fishers, Indiana, for their slain son
• Prominent Syrian exile Shaykh Muhammad al-Yaqoubi flew in from Washington DC to deliver the eulogy
• He called Kassig 'one of our brothers who sacrificed his life for the sake of God'
• The cleric condemned ISIS extremists as 'terrorists' and promised them 'hellfire for every crime their commit'

By Reuters and Snejana Farberov for MailOnline

Published: 17:50 EST, 21 November 2014 | Updated: 02:04 EST, 22 November 2014

Hundreds of friends and family of different faiths came together in an Indiana mosque Friday for a funeral service honoring the legacy of Abdul-Rahman Kassig, formerly known as Peter, who was beheaded by ISIS militants in Syria.

Kassig, a 26-year-old former US Army Ranger, converted to Islam and took a new name during his year-long captivity.

His family said he made a sincere conversion in a process that began before Kassig was captured by Islamic State militants in October 2013, which has seized parts of Iraq and Syria and is the target of intense US-led airstrikes.

Heartbroken: Ed and Paula Kassig, parents of Peter Kassig, reflect as a funeral prayers were held for their son in the mosque at Al-Huda Foundation in Fishers, Ind., Friday

Believer: Abdul-Rahman Kassig, formerly known as Peter, converted to Islam while in captivity in ISIS

Congregation of the faithful: Hundreds of people of different faiths filed in the mosque at the Al-Huda Foundation in Fishers, Indiana, to honor Abdul-Rahman Kassig's legacy

Embraced by community: Ed, right, and Paula Kassig, left center, parents of Peter Kassig, are hugged by Tory Stempf, right center, and his son Morgan Stempf before funeral prayers

Ed Kassig is greeted by a mourner during the absentia prayer service for his son

The Kassigs talk with Lina Midani, of Indianapolis. Paula Kassig wore a white head scarf to honor the customs of her son's faith

Colleague and friend: Paula Kassig is consoled by friend Vanessa Dolder, who has worked as a nurse with her

Powerful eulogy: Sheikh Muhammad Al-Yaquobi, famed Muslim scholar and former preacher of the Grand Umayyad Mosque of Damascus, leads funeral prayers for Peter Kassig, whom he praised as a hero

Men are seen praying for their slain brother, Abdul-Rahman Kassig, who was executed by ISIS in Syria

On Friday, Kassig's loved ones attended the funeral service in the mosque at Al-Huda Foundation in the Indianapolis suburb of Fishers, which included a sermon by a prominent Syrian exile, cleric and Muslim scholar Shaykh Muhammad al-Yaqoubi, formerly Imam of the Grand Umayyad Mosque in Damascus, who flew in from Washington DC to deliver the eulogy.

'He took every risk in order to help the Syrian people, to remove some of their tragedy, to offer them some relief,' al-Yaqoubi said. He called Kassig 'one of our brothers who sacrificed his life for the sake of God.'

He offered condolences to Ed and Paula Kassig, Peter's parents, calling their son a great hero 'who carried in his heart the principles of Islam even before becoming a Muslim.'

Al-Yaqoubi used the occasion to denounce Kassig's murderers, calling them 'terrorists' and declaring that their violent actions go against everything Islam stands for.

‘The question is, is this Islam? Definitely not. Does this group belong to Islam? No, it does not…. Islam is a religion of love, mercy and peace and... encouraging killing is not Islamic,’ he told the audience, according to The Indianapolis Star.

‘Islam and human rights can never clash. God does not preach hatred. God is against oppression,’ reported WTHR.

Al-Yaqoubi promised the extremists who took Kassig's life 'hellfire for every crime their commit.'

The prominent Syrian cleric thanked Peter/Abdul-Rahman for his 'great deeds' and 'acts of championship.'

‘When he went there he knew there were all these risk. He went against all odds showing every type of bravery this is why he’ll be remembered,’ said al-Yaqoubi.

Desperate plea: During their son's captivity, Ed and Paula repeatedly called on ISIS to spare their son, like in a video recorded by them in October

Proof of life: Kassig is pictured here with ISIS executioner Jihadi John, who threatens to behead him in a video for the terror group released earlier this year

Humanitarian: Kassig is shown here working in Syria in a picture posted on a community Facebook page set up to appeal for his release called 'Mercy for Abdul-Rahman Kassig - formerly known as Peter'

The Kassigs sat somberly in the audience during the prayer service, his mother wearing a white head scarf as customary in Islam. Several members of the congregation were photographed approaching the grieving parents to embrace them and offer their condolences.

During their son's captivity, Ed and Paula repeatedly called on ISIS to spare their son, who became the fifth European or American captive beheaded by the militants.

On Thursday, the Kassigs took to Twitter revealing that they had adopted Kassig as a newborn.

'We have always been, and will always be, grateful that his birthmother, Rhonda Schwindt, chose us to be his parents,; they said in a statement. 'We know that she and Peter's siblings, Jana and Sam Schwindt, share in our grief.'

Kassig was a medic and former US Army Ranger who served briefly in Iraq in 2007 in the US Army. He returned to the Middle East in 2012 for a spring break trip while studying political science.

Moved by the suffering of Syrian refugees, he went to Lebanon and volunteered as an emergency medical technician.

Birth family: Peter is pictured here with his biological family, mother Rhonda Schwindt (top right), sister Jana (far left) and baby brother Sam (bottom right)

Peter reached out to his biological family when he turned 18. He formed a close bond with Jana Schwindt (left), whom he called 'Little Punk', and his 'Little Bro' Sam, pictured right

He later founded an aid organization to provide food and medical supplies to refugees from the conflict in Syria, where some 200,000 people have died and millions are displaced.

Another Muslim prayer service will be held for Kassig Saturday in Indianapolis. A third memorial service will be held Sunday at Butler University in Indianapolis, where Kassig had been a student.

Kassig's birth family, mother Rhonda Schwindt, 20-year-old sister, Jana, and 18-year-old brother, Sam, said they will hold a separate private memorial for Peter.

Indiana Gov. Mike Pence has ordered all government buildings in Marion County to fly their flags at half-staff from sunrise to sunset Sunday in Kassig’s memory.