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.

An investigation into the deadly crash of an Azerbaijan Airlines flight in Kazakhstan earlier this week came after the passenger plane experienced “physical and technical external interference,” the carrier said on Friday.