Officials raised the possibilityon Wednesday that the fate of the Malaysia Airlines plane missing for almost a month may never be determined, even as they said its disappearance is now being treated as a criminal investigation.

“Investigations may go on and on and on. We have to clear every little thing,” Malaysia police chief Khalid Abu Bakar said, the Associated Press reports. “At the end of the investigations, we may not even know the real cause. We may not even know the reason for this incident.”