American officials said Tuesday that Mr. McCain’s case highlighted the difficulty of identifying Americans who want to travel to Syria to fight alongside rebels. When the United States faced a similar problem with Somalis several years ago, it was far easier for the authorities to identify those who wanted to travel there to fight because that conflict mostly attracted Somalis. And Somalis live in just a few cities in the United States.

But the Syrian conflict has attracted people from all different ages and parts of the United States — including many with no connection to Syria.

In May, Moner Mohammad Abusalha, a 22-year-old Florida man who had traveled to Syria to join the Nusra Front, died in a suicide bomb attack. He had an American mother and a Palestinian father. A year earlier, Nicole Lynn Mansfield, 33, of Flint, Mich., was killed with Syrian rebels in Idlib Province.

The federal authorities learned only after he arrived in the country that Mr. McCain had traveled to Syria, according to senior American officials. In response, the American authorities included him on a watch list of potential terrorism suspects maintained by the federal government. Had Mr. McCain tried to re-enter the country, he would have almost certainly faced an extra level of scrutiny before boarding any commercial airliner bound for the United States, the officials said.

It is not clear how Mr. McCain was recruited by ISIS and traveled to Syria. According to his Facebook page, he went to Canada and Sweden last year. Many Americans and Europeans who have ended up in Syria have tried to disguise their travels by passing through other countries before heading to Turkey and crossing over its porous border with Syria.

His posts on Twitter, where he went by the name Duale Khalid, give clues to his mind-set. In one message from December 2012, he said that the movie “The Help,” which is about black maids in the South, made him “hate white people.” Other posts disparaged Somalis and gays.

It was on Twitter that he also discussed religion. He said that he was a convert to Islam and that it was the “best thing” that had ever happened to him. “It’s funny to me how all these so call Muslim claim that they love Allah but always curse the one who try to implement his laws,” he said in one post.