WASHINGTON – A derogatory e-mail from a relative led federal authorities to remove a Gambian man from a New York-bound jetliner Thursday during a refueling stop in Puerto Rico, according to a law enforcement source.

The move underscored pressure on airline security officials in the wake of the attempted Christmas Day bombing, when authorities did not act on security concerns that had been expressed by the suspect’s father.

After the man was taken off the plane in Thursday’s incident, the captain said over the intercom that he had been a “serious security threat,” said passenger Joan Mower, director of development for the Voice of America, a government-funded news service.

But federal authorities are no longer convinced of that, the law enforcement source said. When contacted, the relative denied sending the e-mail to U.S. authorities and disputed that the passenger was dangerous.

Delta Flight 215 had taken off from Abuja, Nigeria, late Wednesday and stopped in Dakar, Senegal, early Thursday en route to New York. The Gambian man was detained during what Delta says was an unscheduled refueling stop in San Juan, Puerto Rico. There was no indication of a disruption during the flight, Mower said.

“The passenger was removed from the flight for further questioning and remains in custody,” U.S. Customs and Border Protection said in a two-sentence statement, calling him “a potential person of interest.” A spokesman for the Department of Homeland Security provided no further details.

The law enforcement source, who asked not to be identified because the investigation is continuing, said officials initially planned to talk to the passenger after the plane landed at New York’s John F. Kennedy Airport. But high winds forced the pilot to stop and refuel in San Juan, the source said, and the man was taken off the plane at that time by Customs and Border Patrol agents.

“This all turned out to have no basis,” the source said. The source would not discuss the timing or content of the relative’s e-mail, or why the relative later disavowed it.

Jeffrey Quinones, a spokesman for the Customs and Border Patrol in Puerto Rico, said the passenger had not been charged with any crime.

After the man was removed, Mower said, the captain told passengers over the intercom that there were “armed FBI agents outside the door.”

U.S. officials monitoring airline security have been under pressure since a botched bombing attempt on Christmas Day that federal officials said was carried out by Nigerian Umar Farouk Abdulmutallab, who is suspected of ties to al-Qaida.

Abdulmutallab’s father had contacted the U.S. Embassy to express concerns about his son’s extremist views. Those concerns did not lead authorities to deny him a visa or otherwise track his movements, an omission that drew criticism after he attempted to blow up the Detroit-bound airplane.

Since then, the government’s no-fly list has nearly doubled, from about 3,400 people to about 6,000 people, The Associated Press has reported, citing government officials. Officials do not speak publicly about who is on the list or what criteria are used. The Gambian man in Thursday’s flight was not on the list, a source told The Associated Press.

Jayson Ahern, former acting commissioner of U.S Customs and Border Protection who now works for a consulting firm started by former Homeland Security Secretary Michael Chertoff, said it was not uncommon for authorities to develop information about passengers while they are en route to the U.S., and to detain them upon arrival because of it. The agency has the authority to deny admission to someone deemed a threat, he said.

Detaining passengers upon arrival “is not unique,” said Tom Blank, a former deputy administrator at the Transportation Security Administration now with Wexler and Walker Public Policy Associates. But he said the number of such occurrences has declined as authorities have begun gathering and checking passenger information days before a flight departs.

“Delta followed all required vetting procedures before departure of the flight,” said Susan Elliott, spokeswoman for the airline.