The officials said they believed that Mr. Abusalha, who adopted the nom de guerre Abu Huraira al-Amriki, used a large truck in helping to carry out the bombing in the northern province of Idlib, where he had traveled after spending two months in a training camp of the militant group the Nusra Front in Aleppo. The officials said they thought it was the first time an American had been involved in a suicide attack in Syria…

A Facebook profile of a man with the name Moner Abusalha has pictures that appear to be of the same person shown in the photos on jihadist websites. Many images on the page have Arabic writing and quote the Quran. “Verily, with hardship there is relief,” the profile says on one page. The profile also says he likes basketball, football and Eggo waffles…

Intelligence and law enforcement officials said on Friday that it was easy for American citizens to get in and out of Syria, and that many had traveled there for humanitarian reasons, presenting a challenge in determining who might be planning to carry out terrorist attacks in the United States when they return.

“It’s trying to identify those individuals who have violent intent,” said Andrew McCabe, a senior F.B.I. official. “That, quite frankly, is not easy.”