A senior Ukrainian security official on Thursday described four main theories for why a Ukrainian plane crashed in Iran killing 176 people – including a possible missile strike.

Oleksiy Danylov, secretary of Ukraine’s National Security and Defense Council, said the other possibilities “being studied” are a mid-air collision with a drone or “other flying object,” a blast from a terror attack and a technical failure leading to an engine explosion.

He laid out the theories in a Facebook post as an initial report by Iranian investigators said the Boeing 737-800 had been on fire immediately before it went down.

The crash occurred hours after Iran launched 22 ballistic missiles on two bases housing US troops in Iraq, leading some to speculate that the plane may have been hit.

But an initial assessment by Western intelligence agencies was that the aircraft had suffered a technical malfunction, five security sources — three Americans, one European and a Canadian — told Reuters on condition of anonymity.

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, according to the Washington Post.

Danylov said his team wants to search the crash site for possible remnants of a Russian-made Tor-M1 surface-to-air missile after seeing reports about its possible existence on Iranian social media.

“Our (investigative) commission is talking to the Iranian authorities about visiting the crash site and is determined to search for fragments of a Russian Tor air defense missile about which there was information on the internet,” Danylov told Ukrainian news site Censor.net.

He said that Ukraine’s commission includes specialists who helped in the investigation of the 2014 downing of Malaysia Airlines Flight 17 in eastern Ukraine.

Gen. Abolfazl Shekarchi, a spokesman for the Iranian armed forces, was quoted by the semiofficial 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.

In Washington, a Democrat who attended a classified briefing from Trump administration officials — including Defense Secretary Mark Esper, Secretary of State Mike Pompeo and CIA Director Gina Haspel — said the briefers had no intelligence suggesting the plane was shot down.

The flight, which was bound for the Ukrainian capital of Kyiv, took off from Tehran at 6:12 a.m. Wednesday and was approaching 8,000 feet when it suddenly lost contact with ground control, officials said.

According to a report from Iran’s Civil Aviation Organization, eyewitnesses on the ground and from the crew of another flight nearby reported seeing a fire while the Boeing 737 was still in the air.

“The trajectory of the collision indicated that the plane was initially moving toward the west, but after encountering a problem, it turned to the right and was approaching the airport again at the time of the crash,” Ali Abedzadeh, the head of the Civil Aviation Organization, said in the report, according to the Washington Post.

With Post wires