SHERMAN, Texas (CNN) -- A seventeenth person has died from injuries suffered in Friday's bus crash in northern Texas, a hospital spokeswoman said Sunday.
The damaged bus is hauled from the crash scene on a flatbed truck Friday in Sherman, Texas.
The driver of the bus, which was carrying dozens of Vietnamese people on a church trip, apparently lost control on northbound U.S. 75 early Friday.
The bus smashed into a guardrail before rolling on its side and sliding into a gully. The accident happened near the Texas-Oklahoma state line.
Twelve people were declared dead at the scene, and four others died at hospitals, officials said.
Police estimated that, in addition to the deaths, 33 to 39 of the 54 passengers and the driver suffered mild to serious injuries. Watch what investigators are learning about the accident »
The bus was operating illegally, according to the Federal Motor Carrier Safety Administration. The National Transportation Safety Board has begun investigating the crash.
On Friday, NTSB member Debbie Hersman told reporters that the right front tire of the bus had been retreaded, in violation of safety rules. The blown tire was the only one that was recapped, and it was the only one whose tread separated, she said.
Asked what caused the Goodyear tire to lose its air, she said, "we don't know the answer to that question; that's why we're here."
The bus itself was made in 2002 by Motor Coach Industries.
The trip to Carthage, Missouri, had been scheduled by the Vietnamese Catholic Martyrs Church in Houston, though some of the passengers may have belonged to other churches, Hersman said.
The 52-year-old driver had his commercial driver's license, but his medical certificate had expired, she said.
Late Friday, police were assisting NTSB investigators with gathering evidence and mapping the scene, documenting the location of witness marks and scars on the highway and bridge rail over an overpass, she said.
Preliminary evidence suggests that the bus hit a rail on the right side of the bridge and then traveled 1,809 feet before coming to a stop on its right side in the earthen median between the highway and a frontage road, Hersman said.
"As it dropped off the bridge rail, the bus rolled to the right ... where it came to rest." Watch as the fire chief describes the crash scene »
She noted that the bus was not equipped with seat belts, which are not required on motor coaches.
In a written statement, motor carrier agency Administrator John H. Hill said investigators noted "certain irregularities ... about whether the bus was operating under the name Angel Tours or Iguala Busmex. It is important to note that neither of these domestic entities is authorized to operate as a U.S. passenger carrier in interstate commerce at this time."
He added that the agency identified Angel Tours "as being a high-risk carrier due to safety violations detected during roadside safety inspections and was subjected to an FMCSA compliance review in May 2008."
"This review resulted in FMCSA placing Angel Tours' operations out of service," Hill said. "To date, Angel Tours has not provided the agency with evidence of satisfactory corrective actions to the problems discovered and remains out of service."
In addition, "FMCSA has not granted Iguala Busmex authority to transport passengers because it has failed to fully comply with federal safety requirements."
He said police have been asked to stop any of the companies' buses.
Both companies are owned by Angel de la Torre.
A man who answered the phone at Angel Tours office said the owner was meeting with his lawyers and was unavailable.
Massage therapist Leha Nguyen, 45, was a passenger. She was traveling by herself on the bus when she departed Houston at 8:30 p.m. Thursday for what was to have been her fourth trip to Carthage, Missouri.
After sandwiches were passed around, the group said prayers, and Nguyen began to drift off to sleep in her window seat next to an older woman, four rows behind the driver, she said. iReport.com: Were you there? Send photos, video
About 11:45 p.m., the bus passed Dallas, "and as soon as we passed Dallas we were on the two-way freeway and I feel the bus ride a little bit fast and I have a feeling, not a safe feeling," she said.
She added, "I feel a little bit shaky, but I just let myself at peace and then go to sleep."
Her sleep didn't last long. "As I opened my eyes, I see the chair falling off and I was sitting right below the TV and I felt that somebody was laying on my leg and then right next to me there was a lady, she got her arm really crushed up and on top of her there was another lady, she cannot move."
Although her head hurt, Nguyen -- who left Vietnam in 1975 as a refugee -- was in better shape than many of her fellow riders. She accepted an ambulance ride to the hospital, where she learned that her seatmate had died. Once her scalp was sewn up, she aided in translating for other Vietnamese patients whose English was not as good.
CNN's Susan Roesgen contributed to this story from Sherman, Texas.