A school bus packed with high school students smashed through a guardrail along an overpass Monday and crashed nose-first 30 feet below, killing three teenage girls and injuring at least 30 other people, some critically, authorities said.

A car driven by another high school student apparently came close to or struck the bus, causing it to plow through the concrete rail on the elevated section of Interstate 565 and crash onto Church Street below, said Police Chief Rex Reynolds. Two girls died at the scene, and a third died at a hospital.

Students on the bus, which had no seat belts, were screaming when rescue workers arrived. "They were thrown all over the bus," said Fire Chief Dusty Underwood. Some had to be pulled from the crushed front of the vehicle.

More than 30 students and the bus driver were taken to Huntsville Hospital, which became a hectic trauma center with emergency physicians and staff called in to help as ambulances brought in the severely injured.

Christina Collier, 18, Nicole Ford, 17, died at the scene, police said. A third, Tanesha Hill, 17, died at the hospital, a hospital official said.

"This is a heartbreaking tragedy," Gov. Bob Riley said in a statement.

Frantic parents went to the scene, where some students sat dazed or lay draped in white sheets, or went to the hospital to find their children.

Hospital officials said staff members had trouble identifying some of the more severely injured students who were unable to talk and had no identification on them.

Before help arrived, terrified students tried to climb from the wreckage.

"They were falling on each other. People were screaming, yelling, crying," said LaWanda Jefferson, 16, whose left arm was fractured and face was bruised. "I was scared, panicking and just getting ready to cry."

Some children called their relatives from cell phones shortly after the bus hit the ground, Jefferson among them.

"All I could hear was screaming in my ears," her grandmother, Doris Harris, said. "Screaming and crying and her saying something about the bus."

The bus was taking students from Huntsville's Lee High School to the Center for Technology, where students can receive special science and math credits.

At least three survivors were in critical condition, hospital officials said.

The bus driver was in critical condition, authorities said, though the police chief said hospital workers were able to speak with the driver.

A Toyota Celica driven by another Lee High School student was the only other vehicle involved, and investigators had spoken with the car's driver as well as witnesses, police spokesman Wendell Johnson said.

The damaged car was still at the crash site Monday afternoon.

Jefferson told the AP that before the crash, the passengers had been talking, joking and laughing. Then she saw a car speed by to the right, and suddenly she was flying across the bus, she said.

"The bus went to the side, and I guess it went over," she said. "When it was falling ... I was just glad when it hit the ground."

She said that if there had been seat belts on the bus, "most folks would not have gotten injured like this."

The National Transportation Safety Board is investigating the wreck. The agency has said that school buses are designed to protect occupants without the need for seat belts.