KYIV (Reuters) - Ukrainian President Volodymyr Zelenskiy said on Saturday the causes of a military plane crash which killed 26 people would be investigated promptly and impartially. A military transport plane carrying air force cadets crashed and burst into flames near a highway in northeastern Ukraine on Friday evening. The State Emergency Service of Ukraine said 27 people had been on board the plane - seven crew members and 20 students. Only one cadet managed to survive. “Yesterday, as a result of a terrible tragedy... Ukraine lost 26 of its worthy sons,” Zelenskiy said on Facebook. “I insist that the causes of this plane crash be promptly clarified and that the investigation be objective and impartial.” He declared Saturday a day of mourning. The Antonov An-26 aircraft crashed while trying to land during a training exercise, Ukraine’s Defence Ministry said, adding that the black boxes with flight data had been found. Ukrainian authorities said they were investigating four possible causes of the crash -- a technical malfunction involving the aircraft; improper performance by the crew; improper performance by flight control; and inadequate maintenance. The State Security Service, a Ukrainian law enforcement agency, said the plane crashed five minutes after the crew commander asked to land after the left engine of the twin-engined plane failed. “According to the preliminary information, the cadets did not directly control the aircraft - all flights were performed by the crew commander,” the State Security Service said in a statement on Saturday. The crash happened around 2 km (1.2 miles) from a military airport. The cadets were from the Kharkiv University of the Air Force, which is run by the defence ministry. The government said training flights on similar types of aircraft had been suspended.