SAN DIEGO -- Authorities Sunday were still trying to determine what caused a deadly bus crash that killed three women and sent 17 people to the hospital.
The California Highway Patrol said Sunday the deceased included a 23-year-old woman from Mexico, a 67-year-old woman from Riverside and a 73-year-old woman from Pasadena.
Only one of the injured passengers, a 5-year-old boy, was listed in life-threatening condition, the CHP told FOX 5 Sunday. He suffered a head injury and other serious injuries in the crash, CHP officials said.
The charter bus crashed with 21 people on board just after 10:20 a.m. Saturday morning along southbound Interstate 15 south of state Route 76. The bus was heading to Tijuana from El Monte, Calif., at the time of the crash, Officer Mark Latulippe with the CHP said.
During its trip, the bus -- which authorities said is owned by Executive Lines Inc. -- made stops in Baldwin Park, Pomona and Riverside to pick up passengers, the CHP said.
Authorities said the driver lost control for unknown reasons while driving in rainy conditions in the Pala Mesa area, causing the bus to roll several times before it traveled off the freeway and landed upside down on an embankment.
Several people were thrown from the bus as it rolled. One of the deceased victims was found pinned underneath the bus.
CHP officials said it appeared most passengers were not wearing seat belts.
"The bus is equipped with three-point harness seat belts with every seat," Sgt. Curtis Martin with the CHP said. "We don't know at this point who was wearing seat belts and who was not wearing seat belts."
The driver, a 52-year-old man from Whittier, Calif., sustained minor injuries and was the only survivor who was not taken to a hospital.
"We got a full statement from the driver," Martin said. "The driver was 100% cooperative, and so he's given a full statement and everything. He was not under the influence of any drugs or alcohol."
The other 17 survivors were transported to local hospitals. Initial reports indicated all 18 survivors had been taken to hospitals for treatment of their injuries, but CHP officials clarified Sunday that the driver was treated at the scene and the other 17 individuals were transported.
On Sunday, CHP officials said that aside from the 5-year-old boy, the other individuals who were taken to local hospitals were treated for minor to serious injuries ranging from scrapes and broken bones to lacerations and other various complaints of pain.
The 5-year-old boy is the only one believed to have life-threatening injuries and was the only minor on board at the time of the crash, authorities said. The other passengers ranged in ages from 23 to 75.
Though some possible factors have been ruled out, it's still unclear what caused the crash.
"We don't know exactly why he lost control," Martin said. "It was raining at the time; it wasn't raining very hard, but it was raining, and the road was wet. So he did spin out at one point and went over the side of the road."
Martin said the bus would get a full mechanical inspection, which will include a device on board that may prove especially useful as investigators try to piece together what happened.
"Getting the information off of the event data recorder that was on board -- it's like a black box for an airplane," Martin said. "That's going to be part of the investigation."