One month into the search for Malaysia Airlines Flight 370, and there's still no sign of the Boeing 777. Investigators are focusing on a handful of scenarios that could have led to the airliner's fate, but they admit that we may never know what really transpired in those final hours. Was the plane hijacked? Did a crew member deliberately crash the jet? Is it ... still out there?

A few days after the plane's disappearance, Mashable collected all the possible theories on what happened. One month later, we've ruled out a few of them. Here's the updated list.

Possible

1. It was hijacked by a member of the crew.

Malaysian authorities have ended their investigation into all 227 passengers on board the plane and cleared all of suspicion. This now leaves the plane's crew, including senior pilot Zaharie Ahmad Shah, 53, and his co-pilot Fariq Abdul Hamid, 27, both of whom are still under investigation. Police chief Khalid Abu Bakar told reporters that the criminal investigation was now focused on four possible scenarios, one of which is a hijacking. If it was hijacked, though, what was the plan? That much investigators don't know. Most hijackings end with a ransom exchange or, as we saw on 9/11, a much more public display of destruction. MH370, it appears, ended in the Indian Ocean.

Image: Australian Maritime Safety Authority

2. It was sabotaged by a member of the crew.

Given reports that the plane's communications transponders were deliberately turned off — one after the other — investigators believe sabotage is a possibility. Someone on board seemingly changed the plane's course — and deliberately masked its location from radar systems. The Wall Street Journal's Jon Ostrower says turning off the plane's transponder "would require disabling a circuit breaker above and behind an overhead panel. Pilots rarely, if ever, need to access the circuit breakers, which are reserved for maintenance personnel."

Who turned off the transponders? Investigators don't know, but they believe someone with knowledge of a Boeing 777's cockpit was responsible for the deliberate action.

3. A crew member had a financial or personal motive to crash the plane.

The disappearance of the flight may be the result of psychological or personal problems of someone on board, said police chief Bakar. "Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money," he proposed at a news conference days after the plane vanished. If that's the case, investigators will likely review who's getting paid what from insurance companies — and zero in a suspect from there. As The New York Times reported on April 1, the payout process has already started.

4. The pilot committed suicide.

Though uncommon, there have been 24 cases of in-flight pilot suicide over the last two decades. SilkAir Flight 185 and EgyptAir Flight 990 are the two commonly cited examples of this type of tragedy. In both cases, investigators concluded that the pilots deliberately caused the crashes.

5. A catastrophic event killed passengers and crew, subsequently causing the plane to crash.

Since the very beginning of the search, authorities have said that whatever happened on board the plane unfolded quickly. Satellite data showed that the plane flew off radar for up to seven hours — meaning that the plane continued traveling until its fuel ran out. It's possible that everyone on board was unconscious or dead due to explosive, rapid or slow decompression, which can occur from any number of instances while traveling in an aircraft.

One of the more notorious decompression-related airplane crashes in recent memory was the Learjet crash that killed professional golfer Payne Stewart in October 1999. While investigators never concluded what caused the decompression itself, what's certain is that the pilots were rendered incapacitated due to a lack of oxygen in the plane (which can occur in minutes if a cabin were to slowly lose pressure). The pilots — and their passengers — died of hypoxia, a deadly condition caused by low oxygen conditions. The plane, however, continued to fly on autopilot for hours. It eventually ran out of fuel and crashed into a field.

It's possible this happened to Malaysia Airlines Flight 370, especially given the news that the jet's engines continued to send data for four more hours after the last contact.

Conspiracy theories

It's at Diego Garcia U.S. military base.

Some have speculated that the plane was hijacked and landed at a U.S. military base on Diego Garcia, a small coral atoll in the Indian Ocean. At one point in the investigation, police were looking into Captain Zaharie Ahmad Shah's flight simulator, and reportedly found the small island's landing strip programmed into the computer — suggesting Captain Shah may have practiced a virtual landing. However, Malaysia's acting Transport Minister Hishamuddin Hussein downplayed the rumors on March 16, ruling out the theory that the plane was there and its hijackers were seeking a ransom.

Image: Google Earth

It's on the Afghanistan-Pakistan border

An anonymous source who works somewhere in the "security services," reportedly told Moskovskij Komsomolets, a Moscow-based daily newspaper known for its sensationalist coverage of world events, that a Malaysian passenger named "Hitch" hijacked the plane and landed it safety in Pakistan. The passengers were reportedly taken to the mountainous region to the southeast of the Afghan city of Kandahar, and the plane remains on a small dirt runway with a broken wing — the result of a hard landing. The paper claims that the reason for the hijacking was blackmail: There are 20 "Asian specialists" on board who are now being used "for bargaining with the Americans or the Chinese."

Image: Google Earth

Timeline of MH370's disappearance