The legal ramifications are frightening, she notes. “You don’t want to be responsible for letting in the wrong person.”

Most requests are aggressively persistent: “They can hound us, trying different members in our office via our email conference contact list. It’s as if they sit online all day trolling the Internet for conference venues in the hopes of scoring invitation letters.”

While visas to the United States get the most attention, visa seekers have also targeted events in Europe, or in other first-world destinations. One Nigerian scientist named “John Smith” tried to register for MICCAI 2013, a medial imaging conference held last month in Tokyo.

Mark Turner, president of DotCom Your Event, an online registration site for international conference events, said he’s well aware of this phenomenon.

Of the roughly 120,000 registrations his company handles annually, he estimates they see maybe 600 fraudulent requests for visa letters, with the vast majority coming from Nigeria. Most of his conference events are held in the U.S. or Canada—attractive destinations for visa seekers.

“We believe these visa letters are sold on the street to people hoping to leave the country. Many won’t get far. Most embassies know what they’re doing,” he said.

He added that suspect applications are not hard to spot and are usually quite distinctive. Filled out in capital letters by “Prince” somebody, the registrations are often paid for with stolen credit cards, whose owners have Western-sounding names and live in Europe or the U.S.—a different name and address from the registrant, he said.

“We’ve tracked down some of the owners of these stolen cards. Other times we’ve checked out the application’s history of credit card transactions and discovered this is the third or fourth card that’s been tried. Or, we’ve Googled their supposed company or research locations and discovered the addresses either don’t exist or are located in the middle of an apartment complex,” he explained.

Turner says another telltale sign is that when these particular applicants register for conferences (which can cost in the thousands of dollars), they tend to go for the full package, selecting every possible option in the registration—from seminars, to books, to t-shirts and gala dinners. “This could mean bumping another candidate from a seat they have no intention of occupying,” Turner noted.

There’s a downside, however, to this kind of profiling, and Turner says they have made errors. “We had a Nigerian applicant try to register for an Ontario, Canada-based architecture conference. The thing was—it was a regional event. So I canceled the application. But then later when the conference opened, we got a call from one of its organizers who said she had the Nigerian architect standing in front of her.”

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