NEWCASTLE, Okla. — Four people were injured when an ambulance crashed into a toll booth on an Oklahoma turnpike early Friday and investigators were trying to determine whether the driver fell asleep at the wheel, Oklahoma Highway Patrol spokesperson Sarah Stewart said.

The Jackson County Emergency Medical Services ambulance traveling from the Altus area to Oklahoma City crashed into the toll booth near Newcastle, about 20 miles (32 kilometers) southwest of Oklahoma City, shortly before 3 a.m., Stewart said.

Stewart initially said the ambulance's lights and siren were activated at the time, but later said that was not the case.

A patient and two paramedics who were inside the ambulance were flown by helicopter to an Oklahoma City hospital following the crash on Interstate 44, also known as the H.E. Bailey Turnpike, according to Stewart. But only the patient and one paramedic, the driver, were admitted, while the second paramedic was treated and released.

A worker in the toll booth was taken by another ambulance to a hospital where she was treated and released, Stewart said.

The crash was still under investigation, Stewart said. Among the questions is whether the driver had fallen asleep, she said.