Malaysia said Tuesday flight-data recorders from downed Malaysia Airlines Flight 17 in Ukraine appear to be in "good condition" and its officials will hold on to the so-called black boxes until an international investigation team is formalized.

"At that time, we will pass the black boxes to the international investigation team for further analysis," Malaysian Prime Minister Najib Razak said. (Follow the latest updates on the Malaysia Airlines crash in Ukraine.)

Despite mounting evidence to suggest that the Boeing 777 jet was shot down by an antiaircraft missile over the war-torn Donetsk region, both forensic evidence at the scene and the black boxes remain crucial to understanding how the jet came apart and crashed. Debris could point to clues about where any missile hit.

The devices, which were handed over to Malaysian officials on Monday, would also definitively rule out any equipment malfunction on the jet itself and provide some indication of the unlikely chance that the pilots saw a launch or knew a missile was approaching.

A refrigerated train carrying the bodies and remains of those killed aboard the plane carrying 298 passengers left the conflict zone and reached international forensics specialists in Kharkiv on Tuesday afternoon. International investigators are eager to start work after five days of thorny negotiations that stalled access to the crash site controlled by Russian-backed separatists.