Police said that they had not been able to examine all the vehicles because many caught fire and continued to burn.
The first pileup, in both the eastbound and westbound lanes between Bellefonte and Milesburg, occurred about 11:15 a.m. It involved about 30 tractor-trailers and 20 passenger vehicles and killed at least four people, state police said.
State Trooper David White said 17 people were transported to hospitals and fire crews were brought in from four counties.
A second nearby accident in the highway's westbound lanes involved three tractor-trailers, one hauling a mobile home. The two pileups, about 65 miles northwest of Harrisburg, closed the interstate for about 20 miles.
Authorities had hoped to resume traffic in the eastbound lanes Tuesday evening, but White said firefighters were unable to extinguish the burning vehicles; both lanes will probably be closed until Wednesday.
A third accident Tuesday afternoon, involving about 12 tractor-trailers and several smaller vehicles, led to a shutdown of the westbound lanes between Danville and Limestoneville, about 50 miles from the earlier accidents.
Central Pennsylvania were struck by snow squalls Tuesday that reduced visibility to near zero, said Paul Head, a weather service meteorologist.
