WASHINGTON (AP) — United Airlines stopped a prominent security researcher from boarding a California-bound flight late Saturday, following a social media post by the researcher days earlier suggesting the airline’s onboard systems could be hacked.

The researcher, Chris Roberts, attempted to board a United flight from Colorado to San Francisco to speak at a major security conference there this week, but was stopped by the airline’s corporate security at the gate. Roberts founded One World Labs, which tries to discover security risks before they are exploited.

Roberts had been removed from an earlier United flight Wednesday by the FBI after landing in Syracuse, New York, and was questioned for four hours after jokingly suggesting on Twitter he could get the oxygen masks on the plane to deploy. Authorities also seized Roberts’ laptop and other electronics, although his lawyer says he hasn’t seen a search warrant.

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