State police are investigating a one-vehicle crash early this morning in Franklin Township that left one man dead and another injured, authorities said.

The wreck happened on Route 528 about 2:35 a.m. when the driver lost control of his Chevrolet Envoy and slammed head-on into a tree.

“It was one of the hardest impacts I’ve seen for a while,” said Ken Wilson, chief of the Prospect Volunteer Fire Department.

The front-seat passenger, a man believed to be in his 30s, was killed.

The driver, who was in his late-40s, suffered unknown injuries but he was talking to emergency crews following the crash. A medical helicopter flew him to a Pittsburgh hospital.

A police report was not immediately available.

Neither occupant was wearing a seat belt. Firefighters did not know why, given the impact, the vehicle’s air bags did not deploy. The SUV sustained severe damage.

Butler County Coroner William Young III pronounced the passenger dead at the scene.

The road was closed at English Oak Lane north to Christley Road to allow a police accident reconstruction team to take photographs and measurements of the scene.