US, British, and Canadian intelligence have determined that the Ukrainian plane that crashed in Iran on Wednesday morning was shot down by an Iranian surface-to-air missile — a conclusion Iran has strongly denied.

Physical evidence at the crash site could prove what happened, but images from the scene show workers appearing 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 flight recorders from the Boeing 737-800.

Visit Business Insider's homepage for more stories.

Iran has used bulldozers to move around pieces of debris from a crashed Ukrainian passenger jet, possibly destroying evidence that could help prove what happened to the doomed flight.

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

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

Iran has said the plane crashed because of an unspecified technical issue on board.

But the US, British, and Canadian governments all say the jet was shot down by an Iranian surface-to-air missile, citing information from their intelligence services. A video also appeared 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 that 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 impossible" 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 crash site is among numerous problems that complicate the pursuit of the truth behind the crash.

The Ukrainian jet was manufactured by a US company, Boeing, whose participation in the investigation is in doubt amid a turbulent time for relations between Iran and the US.

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

The Wall Street Journal cited Ali Abedzadeh, the head of Iran's Civil Aviation Organization, as telling Iran's Channel 2 news that the agency didn't plan on handing over the flight-recorder information to Boeing or the US in its investigation.

The investigation is further muddled as a preliminary report from an Iranian investigation indicated that the so-called black boxes aboard the plane — which harbor data and cockpit communications — were damaged and lost parts of their memory, the Associated Press reported.

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

Abedzadeh denied any Iranian responsibility for the crash, The Journal reported, citing Iran's Channel 2. He told the local news outlet 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."