(CNN) -

Nancy Curtis said she wanted proof her son, Peter Theo Curtis, was still alive after being held captive almost two years by Islamist rebels in Syria, she told "ABC World News Tonight."

She said she insisted intermediaries ask what museum he wrote about in his doctoral dissertation.

"It came back the Western Museum and I thought, 'Yes, that's right. Nobody would know that. It's him, he's alive,' " she said.

Nancy Curtis said she knew exactly who to contact after confirming her son had been released.

"Before I even told my daughter I sat down and I sent an e-mail to Diane Foley," she said, referring to the mother of slain American journalist James Foley.

"We've been through so much together and I didn't want her to hear it from the media first."

Peter Theo Curtis, 45, is believed to have been captured in October 2012 and held by al-Nusra Front, a Syrian rebel group with ties to al Qaeda.

"My heart is full at the extraordinary, dedicated, incredible people, too many to name individually, who have become my friends and have tirelessly helped us over these many months," his mother said earlier in the day in Cambridge, Massachusetts. "Please know that we will be eternally grateful."

The United Nations said Curtis was handed over to U.N. peacekeepers in the Golan Heights, which is under Israeli government control, and was given a medical checkup.

White House national security adviser Susan Rice said Curtis was safe, no longer in Syria and expected to be reunited with his family shortly.

He was on his way to Tel Aviv, according to a senior administration official.

Nancy Curtis spoke to her son briefly by phone Sunday.

"He sounded so happy and excited to be free," she said. He told her "I can't believe they let me out."

His cousin Viva Hardigg said, "We've heard that his health appears good. That was very encouraging."

The United States was not involved in negotiations for his release but was aware of private efforts to secure the release, two U.S. law enforcement officials said. It's not known whether any ransom was paid, the officials said.

Curtis' release comes just five days after ISIS militants released a video of one of its militants beheading Foley.

"Particularly after a week marked by unspeakable tragedy, we are all relieved and grateful knowing that Theo Curtis is coming home after so much time held in the clutches of (al-Nusra Front)," U.S. Secretary of State John Kerry said in a statement.

When asked ealier in the day about the death of James Foley, Nancy Curtis began to weep. "I feel so bad for their family. Diane (Foley's mother) has become a good friend of mine, and I'm just so heartbroken for their loss."

Author and journalist

CNN obtained two videos that appear to have been recorded during the late stages of Curtis' captivity. In one, a gun is pointed at his head, and Curtis speaks rapidly, as if under duress.

He gives his name and the date and says he is a journalist from Boston.

Curtis is an author and freelance reporter who writes under the name Theo Padnos. He contributed articles about the Middle East to various publications, including the New Republic, The Huffington Post and the London Review of Books.

He has also published two books: "My Life Had Stood a Loaded Gun," a memoir about teaching literature to young offenders at a correctional facility in Vermont, and "Undercover Muslim: A Journey into Yemen," which investigates Islamic extremism.

He was born in Atlanta and graduated from Middlebury College in Vermont. Curtis holds a doctorate in comparative literature from the University of Massachusetts and is fluent in French and Arabic, according to a statement from his family. He also speaks German and Russian.

"Theo has a deep concern and regard for the people of Syria," his mother said, "which is why he returned during the war. He wanted to help others and to give meaning and to bear witness to their struggles.