Defense Minister, Mario Banozic, said on Monday that he expected that the flight recorder of the crashed MiG-21 fighter jet would help determine the exact cause of the accident.

The old Soviet-era fighter jet, which carried two Croatian Air Force pilots, crashed last week in a remote area of northeast Croatia, during a routine training flight. Both pilots ejected safely and the plane’s ‘black box’ was found amid the debris at the crash site. The cause of accident is unknown, although pilots had reported engine problems.

Banozic told reporters on Monday that one of the pilots who sustained injuries is now recovering in hospital, while the other pilot was already released home with only minor bruising. Investigators combed through the area and the collected debris was transported to Zagreb.

“We expect the flight recorder to provide us with some data and help us determine the cause of the technical problem,” Banozic told reporters, after attending a ceremony at a barracks in Velika Gorica, outside Zagreb, marking the 31st birthday of the Croatian Air Force.

The chief commander of the Croatian Air Force, Brigadier General Michael Krizanec, said that the crashed plane was one of 12 which had underwent a controversial overhaul in Ukraine and were delivered back in 2014-15. The same jet was later sent for repairs again, Krizanec added, without saying where.