He was a carefree Italian with a recent law degree from a Roman university. She was “a totally Virginia girl,” as she puts it, raised across the road from George Washington’s home. Their romance, sparked by a 2006 meeting in a supermarket in Rome, soon brought the Italian, Domenico Salerno, on frequent visits to Alexandria, Va., where he was welcomed like a favorite son by the parents and neighbors of his girlfriend, Caitlin Cooper.

But on April 29, when Mr. Salerno, 35, 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.

Ms. Cooper, 23, who had promised to show her boyfriend another side of her country on this visit — meaning Las Vegas and the Grand Canyon — eventually learned that he 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 while Ms. Cooper, her parents and their well-connected neighbors tried everything to get him out.

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 — problems that can startle Americans as much as their foreign visitors.

“We have a lot of government people here and lobbyists and lawyers and very educated, very savvy Washingtonians,” said Jim Cooper, Ms. Cooper’s father, a businessman, describing the reaction in his neighborhood, the Wessynton subdivision of Alexandria. “They were pretty shocked that the government could do this sort of thing, because it doesn’t happen that often, except to people you never hear about, like Haitians and Guatemalans.”

Each year, thousands of would-be visitors from 27 so-called visa waiver countries are turned away when they present their passports, said Angelica De Cima, a spokeswoman for Customs and Border Protection, who said she could not discuss any individual case. In the last seven months, 3,300 people have been rejected and more than 8 million admitted, she said.

Though citizens of those nations 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, including a hunch that the person plans to work or immigrate, or evidence of an overstay, however brief, on an earlier visit.

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 caught in a bureaucratic tangle or paperwork errors. But despite encouraging officers to resolve such cases quickly, excesses continue to come to light.

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, Erla Osk Arnardottir Lillendahl, 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.

In questioning Mr. Salerno, customs agents seemed to suspect that he intended to work here. Ms. Cooper, a copy editor for an educational publication, said she was in the airport lobby when an agent called to ask about Mr. Salerno’s income and why he visited so often.

The youngest son of a prosperous contractor in Calabria, Mr. Salerno helps out in his brother’s law firm in Rome and is able to visit the United States several times a year. Neighbors said he joined volunteers in refurbishing the Wessynton recreation center in 2006, then became one of its summer attractions, kicking a soccer ball with the kids and playing tennis with the adults.

“He just is a very open, fun and helpful guy,” said Christopher M. Porter, a resident of Wessynton.

Ms. Cooper said that at the airport, when she begged to know what was happening to Mr. Salerno, an agent told her, “You know, he should try spending a little more time in his own country.”

Another agent eventually told her to go home because Mr. Salerno was being detained as an asylum-seeker.

“The border patrol officer said to my face that Domenico said he would be killed if he went back to Italy,” she recalled, voicing incredulity that, in his halting English, he could express such a thought. “Also, who on earth would ever seek asylum from Italy?”