U.S. President Donald Trump claimed Thursday that Iran may be linked to the flydubai co-pilot who tried to crash an Israel-bound plane, as investigators worked to determine why he stabbed the flight's captain, causing the aircraft to plummet before others on board landed it in Saudi Arabia.

Trump was asked by reporters at the White House whether he had been briefed about any links between the pilot behind the attack and Tehran. He said, "We're working on it right now."

"I would say the answer, based on what I'm hearing, is yes, but we're working on it right now," Trump said. He did not offer details.

Asked if the U.S. would retaliate against Tehran if it was behind the attack, Trump said, "Oh they'll be hit, very hard, don't worry."

"You just ask them," he added. "They know what happened. They'll be hit very hard."

Passengers on the flydubai plane – which was carrying 174 people, mostly Israelis, from the United Arab Emirates to Tel Aviv – have described how the bloodied Indian captain managed to open the cockpit door to let them in before they grappled with the co-pilot and stabilized a descent so sharp that the plane's rudder was torn away.

Saudi Arabia said Thursday ​that its initial investigation had indicated ⁠that ⁠the co-pilot, an Omani citizen, had assaulted the captain during Wednesday's flight, forcing the plane to make an emergency landing in the Saudi city of Tabuk.

The ⁠Interior Ministry ⁠said in a statement that the captain and co-pilot, ⁠who were both treated in hospital for injuries, ​left for Abu Dhabi ​Thursday morning after receiving medical ⁠clearance. ‌Saudi ‌Arabia said ⁠it had ‌conducted the preliminary investigation ​with security teams ⁠from the ⁠United Arab Emirates.

It was not clear what the co-pilot used as a weapon.

The UAE government said its own investigation team had begun looking into the attack and "whether it was linked to any terrorist purpose," noting that Emirati authorities have jurisdiction because the plane is registered there and UAE law applies to crimes committed on board.

Israeli Prime Minister Benjamin Netanyahu told Fox News that the suspect apparently got through a loophole in Israeli aviation rules that normally bar pilots from flying to Israel if they are from countries that do not have diplomatic relations with Israel, which Oman does not.

Asked if there is any indication of Iranian involvement, Netanyahu said "it's too early to say."

Netanyahu earlier said he also spoke with the UAE's president, Sheikh Mohamed bin Zayed Al Nahyan, who agreed that Israel would join the interrogation of the suspect.

Trump told reporters at the White House that he and Netanyahu discussed the incident "at length" by phone and it seemed as though the co-pilot "was perhaps a terrorist or perhaps a whack job."

Oman's government has not commented.

Israel's Foreign Ministry said most of the people on board were from Israel, along with others from India and the UAE. The U.S. State Department said four passengers had American citizenship.

Flydubai has said its flights to and from Israel would be suspended. Netanyahu told CBS that Israel plans to fly home anyone stranded in Dubai as a result.

Someone in the cockpit sent out two four-digit codes to discreetly signal distress while the plane flew erratically near the Jordanian border with Saudi Arabia, according to Flightradar24.com. The first code was "7700," the universal "squawk" signal for an emergency, then "7500," which indicates unlawful interference, including a possible hijacking.

The codes are used by pilots when speaking over the radio may not be possible or wise.

Flydubai said Wednesday that the jet was secured by on-duty crew members "who diverted and landed the aircraft safely." A regional official said the crew members were pilots in training. The official spoke on condition of anonymity because they were not authorized to speak to the media.

In the chaos that began about two and a half hours after takeoff, the flight descended from 33,000 feet to 17,000 feet (around 10,000 meters to 5,100 meters) in about a minute, before making an erratic circle close to the Jordanian border and landing in Tabuk, according to Flightradar24. The site identified the aircraft as a Boeing 737 Max 8.

Flydubai said crew members helped secure the plane but released no details about them.

The head of the New Zealand flight school where the Indian captain trained, Philip Kean, called it "miraculous" that the aircraft, which was never designed for that sort of dive, did not break up completely.

During the frantic search for a place to land, a person who heard the flight's communications with air traffic control described the plane being turned away by Saudi Arabia and Jordan before eventually landing in Tabuk in Saudi Arabia while passengers screamed in the background. The pilot's voice was trembling as he called "mayday" and said a hijacking was taking place.

The air traffic controllers gave no reason for the refusals. The person who heard the communications spoke on condition of anonymity because they were not authorized to speak to the media.