With world powers divided over the cause of a Russian jetliner crash, much rests on forensic teams as they scour a sandy trail of wreckage almost a week after 224 people died in Egypt's worst air disaster.
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.”
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.
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.