The Soviet-era Tu-141 reconnaissance drone, which is thought to have been launched from Ukraine, flew over Romania and Hungary before crashing in a Zagreb park on March 10.

All three countries are NATO members.

Nearly 100 parked vehicles were damaged, but no one was injured in the incident, which happened just a few kilometres from the city centre.

The 14-meter (46-foot) drone, weighing more than six tonnes, crashed just next to a students' residence area housing 4,500 people and other residential buildings.

"The ballistics experts' report showed that it was an aircraft bomb" carried by the drone, prosecutor Jurica Ilic told reporters.

The OFAB 100-120 bomb contained no military or commercial explosive but rather an "explosive substance", said forensics expert Ivana Bacic.

The unconventional substance was completely decomposed in the blast, making its identification difficult, she added.

The drone crashed because of a malfunction in its automatic landing system, the experts added.

Although the Croatian government has said that the drone was launched on Ukraine's territory, the investigators refused to answer questions on where it had come from or whether Zagreb was its final destination.

Both Ukraine and Russia have denied it was theirs, Prime Minister Andrej Plenkovic said in March.

Zagreb is about 550 kilometres' (340 miles) flying distance from the border with Ukraine, which Russia invaded in late February.