EXCLUSIVE

The crash of EgyptAir flight 804 into the Mediterranean Sea that killed 66 people was caused by a hot Apple product ... according to a new suit.

The families of several of the victims of the May 19, 2016 crash claim the tragedy was due to the co-pilot's iPhone 6S or iPad mini overheating in the cockpit and catching fire.

According to the docs ... an investigation revealed the device ignited and led to a bigger fire in the cockpit, which ultimately took the plane down.

It should be noted, however -- some industry experts have questioned this phone theory ... and believe the fire on the plane started beneath the cockpit in the avionics bay due to a short circuit or some kind of explosion.

Still, the families believe there's enough evidence to hold Apple responsible for the deaths of their loved ones ... so they're suing for damages. The airline is also listed on the suit.