Domenico Salerno, who was a carefree Italian with a recent law degree from Roman University, and Virginia girl Caitlin Cooper, started their romance by a 2006 meeting in a supermarket in Rome, which soon brought Salerno, on frequent visits to Alexandria, Va.

But on April 29, when Mr. Salerno presented his passport at Washington Dulles International Airport, a Customs and Border Protection agent refused to let him into the United States. And after hours of questioning, agents would not let him travel back to Rome, either. Over his protests in fractured English, he said, they insisted that he had expressed a fear of returning to Italy and had asked for asylum. Then Salerno had been sent in shackles to a rural Virginia jail and there he remained for more than 10 days, locked up without charges or legal recourse.

Mr. Salerno’s case may be extreme, but it underscores the real but little-known dangers that many travelers from Europe and other first-world nations face when they arrive in the United States.

Angelica De Cima, a spokeswoman for Customs and Border Protection, said she could not discuss any individual case and that though citizens of those 27 so-called visa waiver countries do not need visas to enter the United States for as long as 90 days, their admission is up to the discretion of border agents. There are more than 60 grounds for finding someone inadmissible. While those turned away are generally sent home on the next flight, “there are occasional circumstances which require further detention to review their cases,” Ms. De Cima said. And because such “arriving aliens” are not considered to be in the United States at all, even if they are in custody, they have none of the legal rights that even illegal immigrants can claim.

Government officials have acknowledged that intensified security since the Sept. 11, 2001, attacks has sometimes led to the heavy-handed treatment of foreigners. One recent case involved an Icelandic woman who was refused entry at Kennedy Airport because, a dozen years earlier, she had overstayed her visa by three weeks. The woman was deported Dec. 10 after what she described as 24 hours of interrogation and humiliating treatment — locked in a cell and barred from making phone calls. The Department of Homeland Security later issued a letter of regret.