BBC News reports the Federal Bureau of Investigation (FBI) has issued a formal alert telling airlines to be on the lookout for hackers. It follows an onboard tweet from security expert Chris Roberts, who joked about being able to hack into a United Airlines plane's wi-fi network. A terrorist could theoretically take over systems that fly a plane by compromising equipment at their seat. United Airlines has now banned Roberts from all its flights. The FBI and the US Transportation Security Administration (TSA) said they had no information to support claims a plane's navigation system could be interfered via its onboard wi-fi kit, but added that they were evaluating the evidence.