Pilot suicides are uncommon, but there have been several in the past 30 years

The main theory to explain the crash this week of the Germanwings jet in France has shifted to suicide by the co-pilot.

Authorities investigating the crash had already noted that the Airbus A320's relatively slow descent from cruising altitude - it took about 8 minutes - was strange.

Additional information pointing to an intentional act then emerged.

On Thursday, a prosecutor in Marseille said that the German co-pilot appears to have crashed the plane deliberately, Reuters reported. He was left in sole control of the aircraft after the captain left the cockpit but refused to re-open the door and pressed a button that sent the jet into its fatal descent, the prosecutor told a news conference carried on live television.

Various outlets have reported that the pilot first tapped gently on the cockpit door, then pounded, then tried to break the door down gain to access to the cockpit as the plane descended.

Passengers place tremendous confidence in airline pilots. In the Germanwings case, the two men in the cockpit were responsible for 150 people, all of whom died instantly when the plane hit a mountainside in the French Alps at over 400 mph.

Pilot suicides in commercial aviation are exceptionally rare.

However, there have been three incidents in the past 30 years in which suicide was blamed for a crash - and one had eerie similarities to the circumstance that are now being discussed in connection with the Germanwings tragedy.

In 2013, a Mozambique Airlines went down in Namibia, with 27 passengers and crew on board.

According to a Washington Post story on pilot suicides in the aftermath of the the disappearance of Malaysia Airlines flight 370, the Mozambique co-pilot "left to use the bathroom, and when he returned, he found the door shut."

The Post report continued: "Inside, the pilot had switched the plane's altitude reading from 38,000 feet to ground level, IBT reports. Recordings show someone pounded on the door to the cockpit as the plane plummeted. Investigators later concluded the plane had crashed because of 'intentional actions by the pilot.'"

In 1997, SilkAir Flight 185, a Boeing 737, went down in Indonesia. All 97 passengers and 7 crew died. Pilot suicide emerged as the dominant theory to explain the incident.

In 1999, EgyptAir flight 990, a Boeing 767 flying from New York to Cairo, crashed in the Atlantic, killing 217 people. Again, pilot suicide was pointed to as the cause.