As Iran and Western states offer diverging explanations for what caused Ukraine International Airlines Flight PS752 to fall out of the sky just a few minutes after takeoff on January 8, there are growing concerns that crucial evidence may have been compromised.
On the day the Boeing 737-800 carrying 176 people slammed into the ground just outside Tehran and before Ukrainian investigators had even arrived, Iran began using heavy machinery, including bulldozers, to help clear parts of the crash site.
Two days later, on January 10, CBS reported that there were no investigators or security left and that scavengers were already combing the site.
Officials in Tehran have said the crash, which killed everyone on board, was caused by a technical malfunction, while officials in Washington say it was highly likely a surface-to-air missile. Iran's decision to immediately clear parts of the site has raised suspicions it is seeking to cover up evidence and avoid blame for the tragedy.
Annex 13 of the International Civil Aviation Organization (ICAO), a United Nations agency that works with countries and industry groups to help set standards and recommended practices, states that the country of the crash's occurrence should "take all reasonable measures to protect evidence" as well as protect site access from "unauthorized persons, pilfering and deterioration."
WATCH: Investigator Explains Video Evidence
Iran is one of 193 member states of the ICAO. The Montreal, Canada-based organization declined to comment to RFE/RL on whether Iran appeared to be complying with standard practices.
When asked by RFE/RL if wreckage should be removed from such a major crash site on the same day, aviation accident investigator Anthony Brickhouse said, "absolutely not."
"In the first day or so, you don't remove anything. You want everything as sterile as possible in that wreckage," said Brickhouse, who is also an aviation professor at Embry-Riddle Aeronautical University in Florida.
A crash site should be immediately cordoned off by police -- or military, depending on the country -- until investigators arrive, Brickhouse said. In the case of the United States, local police secure the site until investigators from the National Transportation Safety Board (NTSB) or the Federal Aviation Administration (FAA) arrive. Those two agencies will then determine who else gets access to the site.
The investigators will generally first do a walkthrough of the crash site to get a "mental picture" of what possibly happened, Brickhouse said. For a large crash site like a commercial jet, investigators can be at the site for several days.
"Getting on the ground and examining the wreckage is going to be paramount. Because depending on what happened, and depending on how badly the wreckage is damaged, certain signatures should be there," Brickhouse said.
A missile would likely leave shrapnel holes in the fuselage of the aircraft emanating inward as was the case with MH17, the Malaysia Airlines flight which, according to the findings of a Dutch-led investigation, was downed by a Russian-made missile on July 17, 2014, killing all 298 abroad. Damage emanating outward would be an indication of an onboard explosion as was visible in the wreckage of the Pan-Am flight over Lockerbie, Scotland, which was downed in December 1988.
However, such signs wouldn't be conclusive on their own, he said. Among the most important pieces of evidence are the cockpit voice recorder and the data flight recorder. Eyewitness accounts, including any video taken by bystanders, are also valuable in determining the cause.
"You never take one piece of evidence by itself and say, this is what caused the accident. You typically corroborate different bits of information," he said.
At the same time, photographs are taken of the crash site using digital cameras on the ground and drones for a bird's eye view. The geolocations of the pieces of wreckage are also recorded.
More recently, investigators have been using laser scanners attached to tripods to make a digital recording of a crash site for future usage in 3D or virtual-reality format, he said.
After the site is photographed, certain parts of the plane are moved to a secure location for investigators to study further.
According to Annex 13, the country where the crash occurred -- in this case Iran -- has the right to lead the investigation.
Ukraine and the United States have the right to participate as the owner and manufacturer, respectively, of the plane.
Ukrainian investigators, who arrived in Tehran on January 9, have been given access to the cockpit voice recorder and the data flight recorder
Iran initially said it would not allow the United States to participate in the investigation. However, Iran later sent notification of the crash to the NTSB -- the federal agency responsible for investigating all aviation crashes in the United States -- a formal process that then allows the United States to exercise its right to join the investigation.
The NTSB said on January 9 that it had named an accredited representative to the case, but could not say when or if the representative would visit the site due to U.S. sanctions that prevent it from cooperating with Iran's civil aviation authority.
U.S. Treasury Secretary Steve Mnuchin said on January 10 he would give sanction waivers so that Americans can participate in the crash investigation.