Ukraine is investigating multiple possible causes of Wednesday’s plane crash in Iran, including a theory that the plane was hit by an anti-aircraft missile. The Boeing 737-800 went down minutes after taking off from Tehran’s airport, killing all 176 people on board. Oleksiy Danilov, Ukraine’s National Security and Defense council chief, confirmed Thursday that various theories behind the crash are “being studied,” including a possible missile strike, mechanical failure, and an intentional explosion inside the plane. Meanwhile, Iran’s civil aviation authority released a preliminary report Thursday that said the plane had already erupted in flames before it crashed. The investigators also said the crew of the plane tried to turn back to the airport before the plane went down. The crash came just hours after Iran carried out missile strikes on two air bases in Iraq that house U.S. forces, but no evidence has linked the two events.