"Our Highway Safety Unit is investigating, but the preliminary investigation appears to (indicate it was) a tragic accident," O'Brien said.
No arrests have been made, and the investigation is ongoing, O'Brien said.
Brown-Rogers, 14, was taken by ambulance to Albert Einstein Trauma Center, where she later died of her injuries, officials said.
Students at the high school were told the news at 10:45 a.m. during face-to-face meetings with the district's counselors, administrators and department coordinators in homeroom.
"We, as a district, will do everything in our power to fully assist the Brown-Rogers family, and will continue to provide emotional support to all members of our district during this very difficult time," District Superintendent Dr. Darlene Davis said in the message on the school district's website.
Wednesday afternoon, students on their way to field hockey practice stopped to give their recollections of Brown-Rogers.
She was new to the school district this fall, they said, and played on the girls' soccer team.
Sophomore Carrie Morgan said that the team would be wearing black hair ribbons at their next game in memory of Brown-Rogers.
"You expect to come to school every day and see her," said fellow freshman Alexis Grady. "Now she's not here. It was completely unexpected. She was a really nice girl."
When students were told about Brown-Rogers' death, some burst into tears. Others sat in stunned silence, Grady said.
The death is the second to affect the Cheltenham High student body this fall.
On Sept. 7, Cheltenham High School junior Paris Talbert was shot and killed near the Finley playground in the West Oak Lane section of Philadelphia, police said.
On Sept. 11, Saleem Snead, 19, of Philadelpha, was charged with the murder.
Marisa Hobson, another of Brown-Rogers' classmates, said she was "really friendly and always smiling."
"We're all upset, we're mourning," Hobson said.
Veronica Squadrito, a sophomore, said the news of Brown-Rogers' death was a shock.
"You hear about that kind of thing, but you don't expect it," Squadrito said.
Contact Bonnie L. Cook at 610-313-8232 or firstname.lastname@example.org.