Latest from Sky news suggests the MD-87 was on takeoff roll and had to abort takeoff when the Cessna crossed its path, then the MD swerved and crashed into a hangar, and caught fire.
Sadly they say over 100 may have been killed in the MD and 4 people in the Cessna. As you can see above heavy fog at Linate at the time which surely must have been a causal factor - it sounds like the Cessna mistakenly taxied onto the active runway.