Huntsville, Ala. ? A fourth high school student died of injuries she suffered when her school bus nose-dived off an interstate overpass, the police chief said Tuesday.

The bus driver, who was found critically injured on the overpass, was among 15 who remained hospitalized, authorities said. Four were listed as critical.

Police Chief Rex Reynolds identified the fourth victim of Monday’s school bus wreck as Crystal Renee McCrary, 17. Like the other victims, she was a student at Lee High School who was on the bus when a car came up in a side lane and the bus veered over the guardrail and crashed about 30 feet below.

Debbie Hersman, a spokeswoman for the National Transportation Safety Board, said investigators were trying to determine how the driver ended up on the overpass, escaping the devastating impact that crumpled the front of the bus.

Police said the bus, taking students to classes at a downtown tech center, swerved on the overpass, plowed through a concrete barrier and plunged to the street. More than 30 students were injured.

Hersman said at a news conference later Tuesday that the barrier was 32 inches high and was consistent with federal standards.

NTSB investigators had met with officials from Laidlaw, the bus operator, earlier in the day and learned that the vehicle was equipped with a video camera, but that the last trip recorded was Nov. 9.

The bus also was fitted with a device that would not allow it to exceed 68 mph. The speed limit on the overpass is 65 miles per hour, Hersman said.

Laidlaw spokeswoman Tiffini Bloniarz said the driver, whose name is not being released, has worked for the company since May 2001 and has a current commercial driver’s license. She would not comment on his driving record.

The company is contracted by Huntsville City Schools and trains its own drivers, Bloniarz said.

Chief Reynolds said a Toyota Celica driven by another Lee High student apparently came close to or struck the bus, causing it to swerve.

There is “some contact evidence,” Hersman said, noting that investigators were looking for paint transfers between the vehicles.

Authorities have not released the name of the driver in what police said is a criminal investigation. Police spokesman Wendell Johnson said evidence will be presented to a grand jury to decide whether charges such as vehicular homicide or manslaughter are warranted.