TOBYHANNA — State police are trying to determine why a tractor-trailer collided on an eastern Pennsylvania highway with a charter bus taking Italian tourists to Niagara Falls, killing the bus driver and two others on the bus and leaving four people in critical condition.

The crash occurred Wednesday morning on Interstate 380 in the Pocono Mountain region. The bus had left from New York and was about a quarter of the way to its first destination when police said say it collided with a tractor-trailer that had crossed into oncoming traffic.

State police said a second tractor-trailer was involved but they were still investigating what led to the accident.

Italian tour operator Viaggidea said there were 16 people on the bus: 14 passengers, a tour guide and the driver. Police said the driver, Alfredo Telemaco, of New York City, died at the scene along with two passengers.