No-fly list: Chris Roberts, a computer security researcher on his way to give a talk about computer security vulnerabilities at a major conference was told he couldn't fly on United Airlines Saturday, due to comments he'd made on Twitter . 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 a United flight on 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. A lawyer for Roberts said United gave him no detailed explanation on Saturday why he wasn't allowed on the plane, saying instead the airline would be sending Roberts a letter within two weeks stating why they wouldn't let him fly on their aircraft. 'Given Mr. Roberts' claims regarding manipulating aircraft systems, we've decided it's in the best interest of our customers and crew members that he not be allowed to fly United,' airline spokesman Rahsaan Johnson told The Associated Press. 'However, we are confident our flight control systems could not be accessed through techniques he described.' When asked what threat Roberts posed if United's systems couldn't be compromised, Johnson said Sunday: 'We made this decision because Mr. Roberts has made comments about having tampered with aircraft equipment, which is a violation of United policy and something customers and crews shouldn't have to deal with.' Grounded: United made the decision not to allow Roberts to fly on United 'because he had made public statements about having manipulated airfare equipment and aircraft systems'. In this Tweet, Roberts speculates on whether he should give the command to drop a 737's oxygen masks . Hacking: Once onboard, he pondered on Twitter whether he would be able to hack into the flight's onboard computer settings. In a previous Tweet (seen here) he posted pictures containing details of aircraft data . Concern: Roberts was questioned by FBI agents  for four hours. All of his computer equipment, including an iPad, a MacBook Pro, several hard drives and several USB memory sticks were confiscated . Johnson said the airline reached Roberts several hours before his flight to tell him he couldn't fly. But a lawyer for Roberts said Sunday that when his client received that call, the caller would only say he or she was from United, and wouldn't give Roberts a name or callback number. When Roberts then tried calling the number back from his phone's caller ID, it rang instead to a resort hotel, and Roberts assumed it was a prank call, Roberts' lawyer said. In recent weeks, Roberts gave media interviews in which he discussed airline system vulnerabilities. 'Quite simply put, we can theorize on how to turn the engines off at 35,000 feet and not have any of those damn flashing lights go off in the cockpit,' he told Fox News. Roberts also told CNN he was able to connect to a box under his seat at least a dozen times to view data from the aircraft's engines, fuel and flight-management systems. 'It is disappointing that United refused to allow him to board, and we hope that United learns that computer security researchers are a vital ally, not a threat,' said Nate Cardozo, a staff attorney with the San Francisco-based Electronic Frontier Foundation, which represents Roberts. Expert? Roberts' talk that he was about to give was in part about the vulnerabilities of transportation systems . Blocked: United isn't happy about Chris Roberts being on its planes will no longer allow him to fly on its aircraft . Cardozo said Sunday he hasn't seen a copy of a search warrant that would have been used to seize Roberts' electronics, and that he's working to get the devices returned. The FBI declined to comment on the matter Sunday. The Government Accountability Office said last week that some commercial aircraft may be vulnerable to hacking over their onboard wireless networks. 'Modern aircraft are increasingly connected to the Internet. This interconnectedness can potentially provide unauthorized remote access to aircraft avionics systems,' its report found. Roberts took an alternate flight on Southwest Airlines and arrived in San Francisco Saturday evening. He speaks this week at the RSA Conference about computer security vulnerabilities.
Chris Roberts says he sent his tweet because he's frustrated that a security flaw isn't being addressed .
The FBI pulled him from a United Airlines  Boeing 737 .
Roberts said FBI agents detained and questioned him for four hours .
Clams anyone can plug a laptop into the box underneath their seat and reach key controls in the plane, such as engines and cabin lighting .