During the night of Saturday, unknown persons threw two manhole covers from a bridge onto Autobahn 7 near Hildesheim, injuring two people.
One of the manhole covers hit the windshield of a car.
The 52-year-old driver was seriously injured and his 43-year-old passenger suffered life-threatening injuries.
Both were taken to the hospital, police said on Saturday.
The second manhole cover landed on the opposite lane and damaged at least four vehicles there.
Both lanes of the A7 are completely blocked between Hildesheim-Drispenstedt and the Hannover-Süd motorway junction.
The Hildesheim police are now investigating several cases of homicide and have set up a murder commission.