Promising a thorough investigation into last week’s accidental shoot-down of a Ukrainian airliner, Iran said Tuesday it had begun making arrests in the case.

But one of those arrested was the person who filmed the passenger jet being struck by an Iranian missile minutes after it took off from Tehran — key evidence that contradicted Iran’s initial denials of responsibility for the disaster that claimed 176 lives last Wednesday.

Gholamhossein Esmaili, spokesman for Iran's judiciary, said Tuesday that “some individuals” had been arrested during the investigation, giving no further details. But Iran’s semi-official Fars news agency said one of those arrested was the man who filmed the missile striking the airliner shortly after takeoff from Tehran, and posted it online. And Al Jazeera confirmed the report, saying Iran’s Islamic Revolutionary Guard Corps was holding the man for questioning about the events he witnessed.

The announcement of the arrests came as Iran’s President Hassan Rouhani vowed to punish those responsible for the “painful and unforgivable” mistake, and pledged a thorough investigation — even as he blamed U.S. aggression as one of the “root causes” of last week’s disaster.

The U.S. ‘inflamed the atmosphere’

“It was the U.S. that caused such an incident to take place,” he said in a speech in Tehran, adding that Washington had “inflamed the atmosphere and made the situation abnormal, threatened and took our loved ones from us.”

The plane was mistakenly shot down just hours after Tehran had struck U.S. bases in Iraq in retaliation for the drone killing of top Iranian general Qassem Soleimani in Baghdad. Of the 176 people on the Kyiv-bound Ukrainian International Airlines flight out of Tehran last Wednesday, 82 were Iranian, and many others were of Iranian descent. Everyone onboard perished when the plane crashed minutes after takeoff.

Rouhani said a special court would be established to investigate the shoot-down.

“This is not an ordinary case. The entire world will be watching this court,” he said.

“Our people know that this accident was the result of an error and mistake, but who was involved and what circumstances led to the accident?”

Iran’s promises to bring those responsible to justice follows days of protests in the country, amid public fury over the government’s initial attempts to cover up the tragic mistake by its military. Iran initially denied any suggestion that it had shot down the plane, but on Saturday it sensationally reversed course, admitting Iranian forces had shot down the aircraft with a surface-to-air missile due to “human error” amid sky-high tensions with the U.S.

Anger over the plane shoot-down and initial cover-up triggered days of protests in Iran, which featured chants against the country’s Supreme Leader Ayatollah Ali Khamenei, and saw security forces fire on demonstrations in Tehran. Esmaili said Tuesday that about 30 people had been arrested for taking part in the protests, which he described as illegal rallies that had put national security at risk.

He added that Tehran was planning to file a criminal case in the International Criminal Court over Soleimani's killing, which he described as “an act of terrorism.”

“Trump personally has confessed ordering this criminal act, and that is the strongest evidence that a court could have,” he said.

There were also conflicting reports over the status of the investigation, following Iranian promises that experts from affected countries — like Canada, Ukraine, the U.S., and France — would play a role in the investigation.

Esmaili said Tuesday that one of the plane’s black boxes had been sent to France for analysis. But France’s bureau of aviation investigation told NBC News that no black box had been sent, nor was it expected.