ALGONQUIN — The cause of the school bus accident that seriously injured a 7-year-old boy at Neubert Elementary School in Algonquin remains unknown and may never be clearly determined.

That's according to a letter sent home with Neubert students Friday.

The letter explained what the school district does know about the accident:

On Monday morning, as students were arriving at school, the second-grader was walking alongside his bus on the curb. Somehow, he slipped into the roadway. And somehow, the parked bus began to roll forward, its rear wheels running over the boy's lower body, according to the letter.

"This accident has saddened and unsettled all of us," D300 Superintendent Ken Arndt said in the letter.

"District 300 staff members care greatly about our students and their families. We are doing everything possible to fully understand what happened here."

The Algonquin Police Department, Illinois State Police, Community Unit School District 300 and the school district's bus service provider, Durham School Services, all are investigating the accident.

So far, police have found no mechanical problem with the bus, and Algonquin police said the department will not file charges against the driver, who was on the bus at the time but not operating it. But both the small bus and its driver remain out of rotation, according to D300 spokesperson Allison Strupeck.

"We do continue to investigate other aspects of the incident," Strupeck said.

Meantime, the letter encourages parents "to ensure their children are familiar with the safe means of entering and exiting buses, to complement the bus safety instruction D300 provides to students."

And the Neubert Parent-Teacher Organization is collecting items to help brighten the boy's spirits during his recovery.

Donations to "PROJECT CARE PACKAGE" should be delivered by Oct. 30 to the school at 1100 Huntington Drive in Algonquin. Suggestions include cards, arts and crafts supplies, comic and joke books, books about dinosaurs, small toys such as cars or Transformers (his favorite) or major retailer gift cards for stores such as Best Buy, Target or Jewel.

The parents of the boy, who has not been identified by authorities, are quoted in the letter as saying he is "doing as well as could be expected with the injuries he sustained. Thank you for the community's concerns, prayers and thoughts."

He was taken to Sherman Hospital in Elgin immediately after Monday's accident, then transferred to Advocate Lutheran General Hospital in Park Ridge. He underwent surgery Tuesday.