IRS building.JPG

Panorama of the damaged Echelon office complex in Austin, Texas, the day after a suicidal man flew his plane into it on Feb. 19, 2010. (Contributed photo/Wikipedia)

Officials are stepping up their investigation into the pilot and co-pilot of

as the search for the missing jetliner stretches into its second week.

Malaysian officials said Monday the co-pilot of the missing jetliner spoke the last words heard from the cockpit and investigators are considering suicide by the captain or first officer as a possible explanation for the disappearance, Reuters is reporting.

The plane vanished March 8 with 239 people on board. While conflicting reports have emerged during the search, data seems to indicate the plane was diverted off its course by someone with knowledge of flying the Boeing 777.

If the crash was due to a crew member's suicide, it would be an exceedingly rare event. A National Transportation Safety Board study released last month shows eight of 2,758 fatal aviation accidents in the U.S. from 2003 to 2012 involved intentional crashes of the aircraft by a pilot or crew member.

All of the pilots involved in aircraft-assisted suicide were male with a median age of 46. Seven of the eight crashes involved only a single pilot aboard the plane. Four of the eight pilots tested positive for alcohol and two were on antidepressants.

U.S. cases

The NTSB report looked at the circumstances involving those eight cases of airline-assisted suicide. In one case, 45-year-old Douglas Lee Scholl flew his Piper Cherokee straight into the ground in Jackson, Minn. It was later determined he had a long history of depression and past hospitalizations for psychiatric problems, none of which were reported to the FAA.

Another involved a 21-year old helicopter pilot who was celebrating his birthday when he learned his girlfriend was ending their relationship. Patrick Pfeifhofer left the party and, securing a helicopter from his job, flew back to Helena, Montana and crashed it into a field, killing himself.

The deadliest incident occurred in Austin, Texas in 2010 when Andrew Joseph Stack III flew his Piper Dakota into an office building, killing himself and an Internal Revenue Service manager. Thirteen others were wounded.

Stack was upset at the IRS and posted a suicide note online before the crash. There were no drugs or alcohol in his system, the NTSB reports.

International cases

There are international examples of crashes due to suicide, too.

In 1999, U.S. officials said the crash of EgyptAir Flight 990 was due to the co-pilot's suicide. The plane, which crashed into the Atlantic Ocean off Nantucket, was co-piloted by Gameel El-Batouty. He was alone on the flight deck when he switched off the auto-pilot and pointed the plane downward and repeated the phrase "I rely on God," 11 times. All 217 people on board were killed.

Egyptian officials refuted the suicide claim and blamed the crash on mechanical issues.

Similar questions were raised after the crash of SilkAir Flight 185, whichwent down in 1997 during a flight from Jakarta, Indonesia to Singapore, killing 104 on board. U.S. investigators said the plane crash was deliberate but Indonesian officials disagreed. Investigators said the plane fell 35,000 feet into a river in one minute and the cockpit voice recorder had been cut off.

Pilot suicide is also being considered in the November 2013 crash of a Mozambican Airline plane bound for Angola. The crash, which killed 33 people, is still under investigation.