A security expert who was headed to a major conference was banned from boarding a flight to San Francisco after he allegedly made a comment on a Twitter about manipulating aircraft systems.

The computer security researcher, Chris Roberts of One World Labs in Denver, was scheduled to deliver a talk about computer security issues at the conference when he tried to board the United Airlines flight on Saturday, according to a Fox 25 report.

However, before boarding, United informed him he wouldn’t be able to fly because he had “made public statements about having manipulated airfare equipment and aircraft systems,” said United Airlines spokesman Rahsaan Johnson according to the report.

Last Wednesday, Roberts posted on his Twitter account: “Find myself on a 737/800, lets see Box-IFE-ICE-SATCOM, ? Shall we start playing with EICAS messages? “PASS OXYGEN ON” Anyone ? :)”

The comment refers to whether or not he would be able to hack into the onboard computer settings of the flight. EICAS is its onboard communication system. It stands for “engine-indicating and crew-alerting system.”

Roberts was actually scheduled to speak on the vulnerabilities of transportation systems at the conference. He was able to make it there after all by booking a flight on another airline.

Roberts had been informed of this several hours before he was scheduled to board the flight, according to Johnson.

His tweet came after he had boarded a flight from Denver to Syracuse, N.Y. When he arrived, FBI agents removed him and question him for hours. He tweeted after the encounter: “Lesson from this evening, don’t mention planes…the Feds ARE listening, nice crew in Syracuse, left there naked of electronics.”

Roberts had his iPad, Macbook Pro, hard drives, and USB drives all confiscated. He was allowed to keep his iPhone.