Theories about why Germanwings Flight 9525 suddenly dropped from the sky began circulating in the wake of its crash Tuesday, but the reason for one of the worst aviation disasters in France’s history remains unclear. In the search for an explanation, investigators have so far turned up more questions than answers.

Much of the mystery hinges on whether the aircraft experienced trouble midair that led the pilots to descend to a lower altitude. While initial reports indicated  the pilots had issued a mayday alert before crashing in the French Alps, it later surfaced that no such distress call had been made.

Germanwings Flight 9525 departed Barcelona, Spain, at 10:01 a.m. local time and was scheduled to arrive in Dusseldorf, Germany, at about 11:40 a.m. Ground control lost contact with the pilots about 30 minutes into the flight, around the time the aircraft reached its cruising altitude circa 38,000 feet. All 150 passengers and crew members were killed in the accident.

One component of the plane’s black-box flight recorder has been retrieved from the remote and treacherous mountainside, a vital piece of equipment that could shed light on the plane’s condition. However, it will likely take some time for investigators to analyze. Below are five theories about what led to the crash of Germanwings Flight 9525.

1. Technical failure. The Airbus A320 is widely considered one of the safest aircraft models in the world, but it’s not perfect. False air-speed indications were blamed for the 2009 crash of Air France Flight 447, and the aircraft’s autopilot was partly the reason for a recent Lufthansa flight suddenly dropping 4,000 feet in one minute, according to the Associated Press. The plane that crashed Tuesday had undergone maintenance an hour before takeoff, but officials said any issues were resolved.

2. Pilot error. Had the pilots, still unidentified, not noticed that the autopilot may have disengaged and sent the aircraft descending into the mountain? Such scenarios in which pilots become distracted and fail to recognize changes in altitude or direction are rare, but they have happened, as Bustle reported.

3. Terrorist hijacking. This is the least plausible scenario, according to authorities. “A terrorist act is not the main theory we’re working on,” said Bernard Cazeneuve, France’s interior minister. However, he added, “All options are being considered.”

4. Rapid cabin decompression. Sudden depressurization of the aircraft, which could be caused by something piercing the fuselage, can prompt a plane to suddenly descend. At this point, there’s no evidence that Flight 9525 was hit by an object from the outside. If it had been, experts would have expected the aircraft to plummet relatively quickly, the Wall Street Journal reported. Flight 9525 descended for eight minutes.

5. A combination of several factors. The most plausible explanation for why Germanwings Flight 9525 crashed involves multiple influences. A mechanical error could have occurred that the pilots subsequently failed to correct. And a sudden decompression of the cabin chamber could have rendered the pilots unconscious.

Investigators hope the plane’s black-box recorders will provide some answers to all the questions.