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 Britain said on Thursday it believed Islamic State may have downed
the jet, but Egypt said there was no evidence of a bomb and Russia
said it was too early to draw conclusions.
That puts the onus on Egyptian-led investigators to prove or
disprove the theory, with only scattered evidence and Egypt's
tourist economy at stake.
Clues that might reveal whether the plane was deliberately brought
down are flung across 10 miles of desert or potentially concealed in
the dying microseconds of cockpit recordings.
With at least one of the black boxes reported to be damaged, and
cockpit sensors likely to be silenced by any blast, the main focus
will be on understanding the wreckage as well as any evidence
gleaned from bodies of the mainly Russian victims.
To tackle their task an international team including Russia, France
and Ireland is likely to draw on lessons from one of the bloodiest
12 months in aviation over a quarter of a century ago.
In December 1988, 270 people were killed when a bomb brought down
Pan Am flight 103 over the Scottish town of Lockerbie.
In September the following year, a DC-10 flown by French airline UTA
blew up over the Sahara en route from the Chadian capital N’djamena
to Paris, killing all 170 on board.
In both cases, investigators concluded that a device may have been
smuggled aboard but drew a blank from 'black box' recorders whose
power supply was severed by the explosions.
Investigators relied on debris, bags and clothes as well as chemical
analysis to find the subtle imprints of an explosion, according to
the report and people involved in the two probes.
"A first indication comes from distribution of the wreckage and then
you look for potential reasons, and one would have to be a device on
the aircraft,” a former investigator said.
"You are looking for traces of high-energy explosion, such as burns
or scorching and penetration of material into bodies or the
aircraft," he said.
“Close to the device itself you might get traces of gas washing over
the surface.”
SCATTERED EVIDENCE
A UK report on the Lockerbie disaster said the bomb sent a double
shockwave and supersonic shrapnel and gases through the Boeing 747,
contributing to the plane's disintegration.
One of the earliest clues in that probe was evidence of fine
"cratering and pitting" in the aircraft, which speeded up the
criminal investigation, a person involved in the probe said.
Investigators in Egypt and Russia, where most bodies are now
located, will be looking for lacerated clothing, deformed baggage
and burn marks in places otherwise free of fire damage, indicating
fire before the A321 plunged into the ground.
Russia has however started to bury some of the victims. Forensic
experts say detailed examination of bodies would be vital in
detecting evidence of any attack.
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In harsh conditions such as the Sinai peninsula where the Airbus
A321 broke up and fell, evidence can be fragile and end up scattered
well beyond the main crash site.
In the UTA disaster, a team including 60 soldiers faced a 50-mile
trail of tiny fragments, 10 times the size of the main wreckage
field, according to an official French report.
They also had to cope with desert winds and move quickly to make
sure vital evidence was found. Their breakthrough came when soldiers
combing through 60 square km of Sahara found parts containing traces
of explosion.
As with the recent Dutch probe into the downing of a Malaysian jet
over Ukraine, investigators may reconstruct the recovered debris on
a specially constructed frame.
That can help pinpoint an explosion's source by examining the impact
of shockwaves and debris on the fuselage. Engines may be examined
for shrapnel and even paint smears if a suspected blast is thought
to have happened near the front of the plane.
Doubts meanwhile remain over what evidence the black boxes will
supply. Egyptian officials said the voice recorder was damaged and
previous disasters suggest the separate flight data recorder would
have seen its power cut and produced little of value.
But science may help fill in some of the blanks.
On the UTA jetliner, as well as a TWA jet whose fuel tank blew up
over the Atlantic in 1996, cockpit tapes ended immediately after the
blast but still yielded vital clues thanks to a procedure called
"spectrum analysis".
Scientists use such methods to examine the signature of any sounds
picked up in the last microseconds of normal flight.
That helped investigators of the UTA disaster to identify a
revealing sound: not of the blast itself but of the airframe
transmitting shockwaves like a tuning fork.
Investigators may be able to compare any usable fragment of audio to
the frequencies produced by those previous explosions to try to
assess whether they are looking at evidence of a bomb.
(Reporting by Tim Hepher; Editing by Giles Elgood)
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