On the first day of October in 2013, Peter Kassig was leading a convoy carrying supplies to a hospital near Deir al-Zour, Syria. A year earlier, he had founded a humanitarian aid organization called Special Emergency Response and Assistance (SERA), and he had just used donations flowing in from the United States to purchase supplies. He knew eastern Syria was no place for Westerners — or Syrians. Still, he insisted on leading the mission to help refugees of Syria’s civil war.

He was captured at the Islamic State checkpoint in Raqqah, a stronghold for the violent group.

Word soon started to spread. His parents, Ed and Paula Kassig, heard the news through a friend, but the militants threatened to kill their son unless they kept quiet. They requested a media blackout and stayed silent for more than year.

In captivity, Kassig converted to Islam and changed his name to Abdul-Rahman.

“I am obviously pretty scared to die but the hardest part is not knowing, wondering, hoping, and wondering if I should even hope at all,” he wrote in a letter to his parents last June. “If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need.”

In a video released Sunday, the Islamic State beheaded Kassig, 26, the fifth Western hostage murdered by militants on video. It came weeks after the group said it would kill him because of the U.S. bombing campaign in Syria.

On Sunday, his parents asked people not to focus on his murder and, instead, to keep “his legacy alive.”

“We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause,” they said in a statement reported by the Indianapolis Star.

Kassig was an only child who grew up in Broad Ripple, Ind., a small town outside Indianapolis. His father taught science, and his mother worked as a nurse. He ran track and played guitar. And after he graduated from high school, he reportedly joined the U.S. Army Rangers and served four months in Iraq. Then he was honorably discharged for medical reasons.

He got married and divorced. He took some college courses. He trained as an emergency medical technician. Then, in 2012, he told his parents he finally found his calling.

Kassig wanted to help others. During spring break in 2012, he flew to Lebanon while studying political science at Butler University. Then he canceled his return flight home.

“Here, in this land, I have found my calling,” he wrote in an e-mail to friends and teachers, the Indianapolis Star reported. “Yesterday my life was laid out on a table in front of me. With only hours left before my scheduled flight back to the United States, I watched people dying right in front of me. I had seen it before and I had walked away before…. I have run until I could not run any more.”

A certified EMT, Kassig spent several months volunteering at a refugee hospital in Tripoli. Lebanon. Then he moved to Beirut and, at age 24, founded SERA, an aid organization that provides medical training and treatment in areas that other humanitarian organizations can’t reach, including corners of Lebanon, Syria and Turkey.

Those who knew him said it was his drive that set him apart.

“While friends drank beer at bars on Gemmayze Street, Kassig grabbed camping gear and set out for the mountains,” journalist Joshua Hersh wrote in the New Yorker. “He visited the Palestinian refugee camps that dot the landscape around Beirut, thinking about ways to bring solar power and other utilities into those neglected communities. Later, as the war in Syria encroached on Lebanon’s borders, sending desperate and wounded civilians into rural communities in the north, Kassig traveled to Tripoli to volunteer his services at a clinic, suturing wounds and comforting the dying.”

Before his capture, Kassig said in an interview that he hoped he made an impact in some small way.

“How much did I impact the political situation inside Syria? None. How much did I impact the political situation back home? None,” he said in the recording. “But — what I did do is over a period of time, in that hospital, I was able to share a little bit of hope and comfort with some people. They were able to teach me a little bit about themselves that I wouldn’t have known otherwise. And we each were given an opportunity to look at the conflict in a different way.”

After his capture, Kassig remained strong in letters he wrote to his parents, saying, “all in all I am holding my own.”

“I hope that this all has a happy ending but it may very well be coming down to the wire here, and if in fact that is the case then I figured it was time to say a few things that need saying before I have to go,” he wrote. “They tell us you have abandoned us and/or don’t care but of course we know you are doing everything you can and more. Don’t worry Dad, if I do go down, I won’t go thinking anything but what I know to be true. That you and mom love me more than the moon and the stars.”

In October, Kassig appeared in an Islamic State video showing the beheading of British aid worker Alan Henning. The militants announced Kassig would be next, and his parents started speaking out. They released pieces of their son’s last letters and started talking to the news media. They begged for his safe return.

“I am hoping that he will somehow hear of this and of other conversations we’ve had or other times we’ve spoken in public,” Paula Kassig told NBC News. “That way he’ll know that we haven’t forgotten him, we haven’t abandoned him and we certainly do love him.”

On Sunday, his parents said they were “heartbroken” by his death but “incredibly proud” of the life he lived.

“We each get one life and that’s it. We get one shot at this and we don’t get any do-overs, and for me, it was time to put up or shut up,” he told CNN in 2012. “The way I saw it, I didn’t have a choice. This is what I was put here to do. I guess I am just a hopeless romantic, and I am an idealist, and I believe in hopeless causes.”