Azerbaijan Airlines says the jet that crashed in Kazakhstan on Christmas Day experienced “physical and technical external interference,” according to an early investigation, as questions swirled about Russia’s possible involvement in the disaster.

Experts say evidence in the Azerbaijan Airlines plane crash in Kazakhstan points to a possible midair explosion, not an encounter with a flock of birds.

The head of Ukraine’s military intelligence (HUR) on Dec. 27 said the aircraft had been hit by “a Russian Pantsir S1 air defense system on Russian terrain.”