The impact from the car caused the bus to plunge off a highway overpass.
Witnesses say that car came up on a side lane and apparently hit the bus. Both vehicles were on their way to a downtown technical center, where students can receive special science and math credits.
Gary Van Etten, the investigator in charge of the probe for the NTSB, said late Monday that there may have been a malfunction in the steering system of the car. The car's driver told investigators "he was having problems steering the car and they got progressively worse when he lost control of the car," Van Etten said.
"As of yet we have not found anything that is any severe violations or defects in the way of the operations," Van Etten said
Several people who witnessed the event said the car, a Toyota Celica, appeared to have steering problems, Huntsville Police Chief Rex Reynolds told CBS affiliate WHNT-TV in Huntsville.
"There was no deliberate erratic driving on part of the driver of the Celica prior to the accident," said Reynolds. Police said the 17-year-old driver of the car has received death threats since the crash.
The investigation also revealed that the bus driver likely was not wearing a seat belt. The NTSB believes the driver was ejected through the bus loading door before it went over the edge of the overpass. Investigators are waiting for the bus driver's medical condition to improve before they interview him, WHNT-TV reports.
Anthony Scott, the bus driver, remained hospitalized Monday, along with two student passengers, hospital officials said.