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Boeing Co. and U.S. government investigators likely won't be able to investigate the Ukrainian plane crash that killed 176 people, senior U.S. investigative sources said Wednesday.

While international agreements allow Boeing and the National Transportation Safety Board, or NTSB, to take part in investigations of overseas crashes involving Boeing aircraft, the sources said hostilities with Iran and U.S. sanctions against it are expected to block U.S. teams from assisting Iranian investigators.

The Ukrainian International Airlines crash came as President Donald Trump announced further sanctions against Iran in response to the retaliation by Tehran on Tuesday evening for the U.S. killing of Maj. Gen. Qassem Soleimani, head of Iran's powerful Quds Force.

The NTSB said Wednesday that it is monitoring the crash and "is following its standard procedures for international aviation accident investigations, including long-standing restrictions under the country embargoes."

The safety board said it is working with the State Department and other government agencies to determine the "best course of action."

Handover of the so-called black box recorders would be standard protocol in any crash, and Iran could send the boxes to a third country, like France, to read out the cockpit voice and flight data.

Rescue workers search the scene of a Ukranian plane that crashed after takeoff in Tehran on Wednesday, Jan. 8, 2020. Ebrahim Noroozi / AP

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Boeing called the crash "tragic" and offered its condolences to the victims and their families. "We are in contact with our airline customer and stand by them in this difficult time. We are ready to assist in any way needed," it said in a statement Wednesday.

The plane was part of a newer Boeing 737-800 series, which is not among the 737 Max planes that have been grounded since March following two crashes overseas in which 346 people died.

Investigators for the NTSB and Boeing were on the ground following those Max crashes in Indonesia and Ethiopia to look for clues and to assist local investigators.

Investigators in Wednesday's crash would be looking for any signs that a mechanical problem, a missile or a bomb brought down the Ukrainian plane.

U.S. aviation sources questioned why Iran had allowed commercial air traffic to depart Tehran shortly after the country launched missiles against U.S. targets in Iraq.

It remains unclear how the plane went down, but video from the crash site on the outskirts of the capital showed what appeared to be pieces of an aircraft fuselage, an engine and other debris.

Iranian state TV said mechanical issues were suspected, but Ukrainian officials said it was too early to determine the cause.

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Ukrainian Foreign Minister Vadym Prystaiko said 82 of the people on board were Iranian, 63 were Canadian and 11 were Ukrainian, including the nine crew members. Ten were from Sweden, four were from Afghanistan, three were from Germany and three were from the United Kingdom. There were no survivors.

Meanwhile, some of the families of the victims of the Max crash in Ethiopia released a statement reacting to the new crash.

"What a sad day. What sad news. When shall we wake up from this nightmare?" the families said through a lawyer. "The news is heartbreaking after almost 10 months of feeling the loss of our families. Some of us don't even know how to feel about this news."

Paul Njoroge of Canada, who lost his entire family in the crash of the Boeing jet in Ethiopia on March 10, said news of the Ukrainian crash "brought a chill in my entire body."

"I know and feel the pain of losing loved ones in such a tragic manner. My sincere condolences goes to the families who lost loved ones in the crash of PS752," Njoroge said.