2. Terrorist hijacking This theory was prominent early on after it was discovered that two Iranians on board — one 18, the other 28 — were traveling on stolen passports.
Investigators haven't found anything linking either to terror groups; it is believed they were trying to illegally immigrate to Europe.
Ever since 9/11, it's much harder for an unauthorized person to enter the cockpit. Cockpit doors have been reinforced and procedures have been put in place to ensure nobody gains entry when a pilot exits. And passengers and crew have shown a willingness to confront anyone trying to take over or damage a plane.
Could someone have been allowed into the cockpit? It's against protocol, but does happen. Back in 2011, Flight 370's co-pilot and another pilot invited two women boarding their aircraft to sit in the cockpit for an international flight.
During the journey, the pilots smoked and flirted, one of the women said this month.
Still, no credible group has taken credit for the disappearance and intelligence agencies say they haven't noticed any chatter in terrorist circles regarding the jet.