A German security expert has been turned away at the U.S. border by immigration officials while on his way to present at the Black Hat conference in Las Vegas.

Thomas Dullien, who's also known by the nickname Halvar Flake, was denied entry into the United States because he was contracted to speak at Black Hat as a private individual and not as a representative of his company, according to a post on his blog. He was refused entry after customs officials found training materials he had packed in his luggage, it said.

Dullien, the CEO and head of research at Sabre Labs, was scheduled to conduct a two-day training course at Black Hat on analyzing software for security vulnerabilities. The session, set to start on Monday and limited to 18 attendees, was sold out.

"I was denied entry to the U.S. for carrying training materials for the Black Hat trainings, and intending to hold these trainings as a private citizen instead of as a company," Dullien wrote on the blog, noting that he has spoken at Black Hat for seven years without any problems.

After an interview that lasted four hours, a U.S. immigration officer told Dullien he required an H1-B employment visa to speak at the conference, the blog post said.

Dullien was forced to return to Germany, where he plans to apply for a U.S. business visa.

"I need to wait until the American consulate opens, and then apply for a business visa. I have not been able to determine how long this might take -- estimates from customs officials ranged from '4 days' to 'more than 6 weeks'," he wrote.