Iran is using bulldozers at the Ukrainian plane crash site, which could make it impossible to find out what really happened

• A Ukrainian airplane crashed shortly after taking off in Iran on Tuesday, killing all 176 passengers onboard.
• US, British, and Canadian intelligence later determined it was shot down by an Iranian surface-to-air missile, which Iran has strongly denied.
• Physical evidence at the crash site could prove what really happened. But images from the scene show workers seeming to move chunks of debris with heavy machinery.
• One expert said this could make it "next to impossible" to properly investigate the crash.
• Iran has also been reluctant to hand over the black boxes from the Boeing 737-800.
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Iran has used bulldozers to move around pieces of debris from a crashed Ukrainian passenger jet, potentially destroying evidence which could help determine exactly what happened to it.

Images and reports from the crash site, just outside Tehran, show at least one bulldozer working in the debris at the site, where the Boeing 737-800 crash-landed on Tuesday, killing all 176 people on board.

The crash site is at the center of a tense geopolitical struggle over exactly what happened to the jet, which was operated by Ukrainian International Airlines.

Iran maintains that the plane crashed due to a technical issue on board.

But the US, British, and Canadian governments all say that the jet was in fact shot down by an Iranian surface-to-air missile, citing information from their intelligence services. A video also appears to show the impact.

Physical evidence from the crash site could provide more information about what happened. But the usefulness of the evidence could be compromised if it is disturbed.

Giancarlo Fiorella, a researcher for the investigative website Bellingcat, shared a thread of photos which he said showed heavy machinery at work.

"I find these photos distressing because this could potentially be the scene of a crime," Fiorella told Channel 4 News.

"If this was a shoot-down event, you don't want to disturb the crash site before a thorough investigation can be conducted, and I'm not sure that one has been conducted."

Eliot Higgins, another Bellingcat investigator, tweeted that disturbing the wreckage would make it "next to impossibe" to properly investigate the crash.

Others, including the US ambassador to Germany, effectively accused Iran of mounting a cover-up.

The presence of heavy machinery at the scene of the crash are among a number of other problems that complicate the pursuit of the truth behind the crash.

The Ukrainian jet was manufactured by Boeing, calling for US participation in the investigation among a thorny relationship with Iran.

It is not yet clear if the US will be allowed access to the crash site to conduct their own investigation, but CNN reported that Iran invited the US to the investigation as "the manufacturer of Boeing to be present," citing the country's semi-official Fars News Agency.

The Wall Street Journal reported that Iran's Civil Aviation Organization doesn't plan on handing over the black box information to Boeing or the US in its investigation, organization chief Ali Abedzadeh told Channel 2 News.

The investigation is further muddled as a preliminary report from an Iranian investigation revealed the "black box" aboard the plane - which harbors data and cockpit communications - were damaged and lost parts of its memory, The Associated Press reported.

The report also claimed that "no radio messages were received from the pilot regarding unusual situations," and eyewitnesses recalled seeing the plane engulfed in flames prior to the crash, AP reported.

Iran's Civil Aviation Organization Ali Abedzadeh denied Iran's hand in the crash, The Journal reported, citing Iran's Channel 2 News. He told the local news outlet that it was "not possible" an Iranian missile had hit the jet.

"How would the antiaircraft system shoot it?" Abedzadeh said. "It wasn't a security area."

• Read more:
• The Ukrainian flight that crashed in Iran was 'highly likely' shot down by a missile, according to reported US intelligence
• Iran official news agency appears to confirm Ukraine jet crash with photo of aftermath
• A Ukrainian airplane crashed shortly after takeoff in Iran, killing all 176 people on board