A Boeing 737-800 passenger jet crashed shortly after take-off and crashed in Iran on Wednesday morning.

The Ukraine International Airlines plane had been flying from Tehran to Kyiv.

Iranian investigators have found one of the plane’s two black boxes, but the head of the country’s civil aviation authority has refused to hand it over to Boeing, the local Mehr news agency reported.

Ali Abedzadeh, the civil aviation chief, added that he isn’t sure where to send the box to analyze the data, Mehr added.

Black boxes record flight data and audio from the cockpit, and often provide valuable information on how plane crashes occur.

Boeing has faced heavy scrutiny in recent months over two plane crashes involving its 737 Max jet, which also took place shortly after take-off. The 737-800 and 737 Max are distinct models of the plane.

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Iranian authorities are refusing to hand over the black box of the crashed Ukrainian jet to Boeing, the plane’s manufacturer, according to Iranian media.

Crash investigators had found one of the two black boxes from the Boeing 737-800 that crashed near Tehran around 6:20 a.m. Wednesday morning local time, Iran’s state media reported.

Black boxes record flight data and audio from the cockpit, and often provide valuable insights into how a crash took place.

The head of Iran’s civil aviation authority, Ali Abedzadeh, said Iran will not give black box to Boeing, the semi-official Mehr news agency said, according to Reuters. He did not appear to specify a reason.

Abedzadeh also said he wasn’t sure where Iran would send the box to analyze the data, Reuters cited Mehr as reporting.

Boeing has not yet responded to Business Insider’s request for comment on the Iranian refusal.

The plane, operated by Ukraine International Airlines, had been flying from Tehran to Kyiv. All 176 people onboard are dead.

The crashed Boeing 737-800 jet was just three years old and underwent scheduled maintenance checks on Monday, two days before the crash.

Data from the flight-tracking service FlightRadar 24 appeared to show the plane departing from Tehran, with communication being lost approximately three minutes later.

The Boeing 737-800 is the previous generation of the manufacturer’s 737 family of jets.

The current generation, the 737 Max, has been grounded worldwide since last March following two fatal crashes caused by a flawed flight-control system known as the Maneuvering Characteristics Augmentation System, or MCAS.

Boeing tweeted in a Wednesday statement: “This is a tragic event and our heartfelt thoughts are with the crew, passengers, 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.”