One person was killed and three were seriously injured yesterday when an ambulance crossed the median of Interstate 70 in Ellicott City and collided with a flatbed tractor-trailer, closing the freeway for nearly four hours.

The ambulance, transporting a pregnant woman to a Baltimore hospital, was headed east on I-70 near Route 29 about 12:40 p.m. when it crossed the grassy median and crashed into a westbound truck carrying lumber, police said. A nurse in the back of the ambulance was ejected and thrown to the pavement.

The truck then collided with an empty box truck and spilled its lumber across the westbound lanes, said Sgt. Thornnie Rouse of the state police.

The ambulance driver, Jeanette M. Roseberry, 27, of Boonsboro, Md., was pronounced dead at the scene. State police helicopters took three people to Maryland Shock Trauma Center in Baltimore: the pregnant woman, Maria M. Williams, 22, of Hagerstown, Md.; nurse Robin A. Banfe, 31, of Smithsburg, Md.; and ambulance attendant John H. Bagley Jr., 34, of Hagerstown. They were listed in critical condition last night, Rouse said.

Neither truck driver was injured, Rouse said.

After the collisions, police quickly shut down all lanes of I-70 west of St. John's Lane to allow helicopters to land.

About 1:30 p.m., an eastbound lane was opened. The westbound lanes remained closed until about 4:30 p.m., with traffic rerouted to Route 29, police said.

Rouse said investigators were trying to determine what caused the ambulance driver to lose control. He said they hoped to gain some insight by interviewing the ambulance attendant, who had been riding in the front passenger's seat.

Rouse said the ambulance was carrying the pregnant patient from Washington County Hospital in Hagerstown to Johns Hopkins Bayview Medical Center in Baltimore. It was not displaying emergency lights or sounding its siren, he said.

Maryland State Police investigate after an ambulance crossed the highway median and collided with a lumber truck.