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.
