US officials said Thursday it was “highly likely” that an Iranian anti-aircraft missile mistakenly shot down the Ukrainian plane that crashed moments after takeoff from Tehran, killing all 176 people aboard, without sending a distress signal, according to reports.

The Boeing 737-800 was struck by a Russian-built Tor M-1 surface-to-air missile, known to NATO as the SA-15 Gauntlet, Newsweek reported, citing a Pentagon official, a senior US intelligence official and an Iraqi intelligence official who spoke on condition of anonymity.

The shootdown was accidental, one Pentagon and one senior US intelligence official told Newsweek, coming hours after after Iran fired 22 ballistic missiles at two bases in Iraq housing US forces in retaliation for the US killing of Quds Force commander Qassem Soleimani.

US intelligence picked up signals of a radar being turned on, sources told CBS News. American satellites also detected two surface-to-air missile launches shortly before the plane blew up, the network said it was told.

A source who attended an intelligence briefing by federal officials Thursday told CBS it appeared that missile parts were found near the crash site.

President Trump, speaking at the White House, said the plane, which was already on fire while airborne, could have been downed by “mistake” while flying in a “pretty rough neighborhood.”

“Well, I have my suspicions. I don’t want to say that, because other people have those suspicions also. … Somebody could’ve made a mistake on the other side,” the president told reporters. “Some people say it was mechanical. Personally, I don’t think that’s even a question.”

Russia delivered 29 Tor-M1s to Iran in 2007 as part of a $700 million contract signed in December 2005.

Gen. Abolfazl Shekarchi, a spokesman for the Iranian armed forces, was quoted by the semi-official Fars news agency as saying that “the rumors about the plane are completely false and no military or political expert has confirmed it,” adding that the rumors were “psychological warfare” by the government’s opponents.

Meanwhile, despite earlier reports that the US would not be involved in the probe, the National Transportation Safety Board has been invited by Iran, through the International Civil Aviation Organization, to assist, two sources familiar with the matter told the New York Times.

Ukraine International Airlines Flight 752 crashed minutes after takeoff from Tehran Imam Khomeini International Airport en route to the Ukrainian capital, Kiev. A security camera captured the impact, which sent burning debris flying across a wide swath.

Images that circulated Wednesday appeared to show fragments of a Tor M-1 missile said to have been found in a suburb of Tehran.

Ukraine Security Council secretary Oleksiy Danylov said Thursday that contact with a Tor M-1 system was among the possible causes for the plane’s crash. The others were a midair collision with a drone or “other flying object,” a terror attack and a technical failure leading to an engine explosion.

A London-based global information company also said the flight was mistakenly shot down by an Iranian missile.

“Photographs purportedly taken near the site of the crash and circulated on social media appear to show the guidance section of an SA-15 Gauntlet short-range, surface to air missile, which landed in a nearby garden,” the firm IHS Markit said in a report, according to USA Today, adding that it could not confirm the authenticity of the images but “assesses them to be credible.”

In a preliminary report, Iran’s Civil Aviation Organization said the crew did not declare an emergency and were trying to return to the airport when the plane went down. Iranian officials have said the plane apparently suffered engine failure.

But IHS Markit said that publicly available air traffic data was “not consistent” with Iran’s claim, adding that the flight data shows a normal ascent until the plane disappears at 8,000 feet.

“This is consistent with a catastrophic incident onboard the aircraft,” the report said, according to USA Today.

“A pilot of an airliner that took off from Tehran airport shortly after UIA Flight 752 told an IHS Markit source that he watched the aircraft take off and then explode in midair,” the report added.

Iran officials have said they have recovered the plane’s “black boxes,” the audio and data recorders, but they were damaged.

Ali Abedzadeh, head of the Iran Civil Aviation Organization, has said Iran would not share the devices with Boeing, though other countries have been invited to take part in the probe in accordance with international guidelines, the Washington Post reported.

“The cause of the accident will not be discovered and announced until the black box is analyzed,” he said.

Under the guidelines outlined in the International Civil Aviation Organization, the country where a crash occurs leads the probe and is responsible for releasing information about incident. Officials in other countries may be included to offer technical and investigative assistance, the Washington Post reported.

Ukrainian President Volodymyr Zelensky said his Iranian counterpart, Hassan Rouhani, has assured him of full cooperation in investigating the crash and that Iran would provide experts access to all data.

A statement on the Ukrainian presidential website says: “The Iranian party assured full cooperation with a view to holding an objective investigation and finding out the causes of the tragedy. Hassan Rouhani stressed that Iran would provide the Ukrainian expert group with prompt access to all the necessary data.”

Also Thursday, Canadian Foreign Minister François-Philippe Champagne spoke by phone with Iranian Foreign Minister Mohammad Javad Zarif and emphasized that Canada, which lost 63 nationals in the crash, wants to be part of the investigation.

Because Boeing planes are manufactured and certified in the US, American officials have the right to take part in the investigation under international rules. But due to sanctions imposed on Iran, there are additional challenges, according to the Washington Post.

Government agencies, including the NTSB, must obtain a license from the Office of Foreign Assets Control to be allowed to travel to the country, a process that experts say can take months or even years.

Meanwhile, a Ukrainian plane carrying 45 experts and search-and-rescue personnel arrived in Tehran on Thursday to take part in the probe as well as identify and repatriate the bodies of the 11 Ukrainians aboard.

With Post wires