A fourth teenager died Tuesday from injuries sustained when a high-school bus careened off a highway overpass in Huntsville, Ala., and plunged 30 feet onto the ground below.

The Huntsville police chief identified the student as 17-year-old Crystal Renee McCrary.

The National Transportation Safety Board is investigating the crash as officials consider whether a teenage driver from the same school may have been at least partly to blame.

Huntsville Police Chief Rex Reynolds said an orange Toyota Celica driven by another Lee High student on apparently came close to or struck the bus, causing it to swerve. The bus plunged 30 feet from an Interstate 565 overpass Monday afternoon, landing nose first, killing two girls and injuring more than 30 other students. A third girl died later at the hospital.

Reynolds declined to say whether charges would be filed against the Lee High School student driving the Celica.

There is "some contact evidence," Debbie Hersman, a member of the National Transportation Safety Board, said, noting that investigators were looking for paint transfers between the vehicles.

During a press conference Tuesday, Hersman said NTSB will look into whether there was a camera onboard the bus, or other instruments that will determine how fast the bus was going at the time of the incident. Investigators will also look for cameras in the surrounding area that may help them piece together what happened, and will interview both first responders and other passengers. They will try to put together a seating chart as to who sat where on the bus, and how people moved around in the bus during the incident.

"We're going to try to do as much as we can today but we will continue our work on scene until it is completed," Hersman said.

From her window seat in a school bus packed with laughing classmates, LaWanda Jefferson spotted the car seconds before she felt herself catapulted sideways.

"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."

"They were falling on each other. People were screaming, yelling, crying," said Jefferson, 16, who suffered fractures to her left arm and cuts and bruises to her face.

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

Five people, including the bus driver, had undergone surgery, a hospital spokeswoman said.

The driver was thrown from the bus and found atop the overpass, and was in serious condition Tuesday, Hersman said in a cable news interview.

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 below.

Students on the bus, which was not equipped with seat belts, were screaming when rescue workers arrived. "They were thrown all over the bus," said Huntsville Fire Chief Dusty Underwood.

Some had to be extracted from the crumpled front of the bus, he said.

The police chief identified the high school students who died at the scene as Christine Collier, 18, and Nicole Ford, 17. A third, Tanesha Hill, 17, died at the hospital from her injuries, Huntsville Hospital CEO David Spillers said.

Some parents were called to the scene by wailing children on cell phones. Many were angered that police held them back or had no information. At the hospital, some collapsed in tears amid more confusion.

Hospital officials said the horror of the wreck was compounded by the inability of hospital staff to identify some of the more severely injured students who were unable to talk and had no identification on them.

The police chief said the driver and a passenger in the Celica went to a hospital following the crash, but he was not aware if they were treated for injuries. He said the driver was interviewed by police.

The bus driver was in critical condition, said Brooke Thorington, a spokeswoman for the state Department of Education.

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

The NTSB has said that school buses are designed to protect occupants without the use of seat belts. A new design uses strong, well-padded, high-backed seats, closely spaced together, the NTSB has said.

However, Hersman said at a news conference Monday night that the board last week added school bus safety to its list of most wanted transportation safety improvements. She said the board is recommending that new standards be devised to improve safety when buses are involved in rollover crashes.

The Associated Press contributed to this report.