It was not immediately clear what caused the crash, which occurred hours after an Iranian missile attack on U.S.-led forces in Iraq.

Ukraine set up a commission to investigate the plane crash, while UIA said the aircraft had been in good working order and that it was doing all it could to establish what happened.

Founded in 1992 after the collapse of the Soviet Union, UIA operates out of Kiev’s Boryspil airport. Planes operated by the loss-making, privately-held UIA have suffered technical problems in flight over the years but have never crashed before.

UIA officials told a news conference on Wednesday that the Boeing 737-800 involved in the fatal crash was one of the best planes they had and that its pilots were very experienced.

Airline officials said there was no sign that anything was wrong before the plane took off from Tehran’s Imam Khomeini airport and that it had last been routinely serviced on Jan. 6.

The plane, which the company said had mostly been carrying passengers planning to transit via Ukraine to destinations in Europe, was manufactured in 2016 and was acquired directly from Boeing, officials said.

UIA says on its website it had been awarded the IOSA – the IATA Operational Safety Audit certificate – meaning its operational and safety standards were fully in line with international requirements.

The airline, which operates domestic and international flights, has a fleet of 42 planes made up, according to it own website, of various Boeings, including 737-800s and 737-900s. It also operates Embraer aircraft.

Financial problems forced it to scale back its route network last year.

UIA is in the process of trying to modernise its fleet and has ordered three Boeing 737 MAX aircraft, which it has not yet taken delivery of due to continuing safety concerns over the MAX project. (Reporting by Kiev bureau Writing by Andrew Osborn Editing by Andrew Heavens and Gareth Jones)