She was identified as Crystal Renee McCrary, 17. The three others killed were Christine Collier, 18; Nicole Ford, 17; and Tanesha Hill, age unknown. Collier and Ford both died at the scene Monday, while Hill died hours later at Huntsville Hospital.
Fourteen students and the bus driver remain hospitalized, including at least three in critical condition.
National Transportation Safety Board investigators are on the scene trying to determine what caused the accident.
The bus, carrying 40 students from Huntsville's Lee High School, fell 30 feet to the street below an overpass on Interstate 565 and landed on its front end before flipping over.
The bus driver was either ejected or escaped from the vehicle before it fell, NTSB investigator Debbie Hersman said Tuesday. He was found on the overpass by rescue workers, she said.
The initial investigation indicated that a small sedan driven by a fellow high school student veered into the bus' lane, causing the bus to drag along the concrete barrier for more than 100 feet before plunging off the interstate overpass.
Police have interviewed the 17-year-old driver of the orange 1990 Toyota Celica as well as the bus driver.
They planned to interview the bus driver again on Tuesday, Huntsville police spokesman Wendell Johnson said.
According to Hersman, the teen driver told police something went wrong with the Celica that caused it to drift into the bus' lane. The NTSB is investigating whether the car struck the bus.
The car had two flat tires after the accident, although it was unclear when they went flat and what role that might have played.
Photos of the car on the overpass showed damage to the driver's side door and the front passenger side.
Police have not decided whether the teen driver will be charged.
"Huntsville has never seen an accident like this involving students," said Johnson, the police spokesman.
Debate over whether school buses need seat belts
Johnson noted that the bus was not equipped with passenger seat belts or air bags.
"The students, when they were hurled over the side of I-565 ... they were just tossed around inside the school bus," he said.
While most school buses do not have safety belts for students, Hersman stressed that the vehicles are "inherently very safe" and "do very well in most accidents."
"We have generally seen less than 10 children a year killed in accidents on school buses, but one of the issues that have come up is whether there should be restraints," the NTSB investigator said at a news conference Monday in Huntsville.
There are no federal requirements, but some states have demanded that any new school buses purchased by state governments must be equipped with safety belts.
Hersman said the NTSB has not made a decision on whether it supports requiring safety restraints on school buses.
However, the board has been pushing the National Highway Transportation Safety Administration to carry out tests to determine if safety belts should be among a set of performance standards set by the federal government.
Currently there are no national performance standards for school buses.
In addition to determining the cause of Monday's fatal school bus accident, Hersman said the NTSB will be looking at whether it could have any effect on school bus safety regulations.
"Certainly this accident is a tragedy, and we wish that we were not here, but we will do everything that we can to make sure if there is any safety improvement that can come out of this accident," she said. "That is our purpose."
