Two 10-year-old Houston boys could face up to 40 years in prison if convicted of sexually assaulting an 8-year-old boy on a school bus.

The boys were arrested last week after authorities reviewed a security videotape from the bus, Fox affiliate KRIV-TV reports. 

The alleged incident happened in August, but the investigation took several months. Houston police spokesman Kese Smith said Tuesday it is not unusual for investigations involving juveniles to take that long.

Houston school district spokesman Jason Spencer said officials were first made aware of the alleged assault when the district's transportation department was asked by a parent on Aug. 30 to review conditions on a bus that transports special-needs students to an elementary school.

The eight-year-old's mother told KRIV that her child has special needs, as do his alleged attackers. She said she has since removed her child from Thompson Elementary -- the school that all three boys attended at the time of the incident in August.

The alleged victim's mother told the station that school officials should have reacted more quickly to the incident.

"When they notified me, my son was already at home sick," she said. "They sent a letter home saying he was being suspended from school for inappropriate behavior on the bus."

The Houston Independent School District released a statement last week, saying that the inappropriate behavior had been identified during a review of surveillance video tapes on school buses.

"One of those videos, filmed on August 25, 2011, appeared to show three male students engaged in inappropriate activity," the statement said, adding that parents, police and protective services were advised on August 30.

Bill Hawkins, a juvenile defense attorney who previously ran the juvenile division of the Harris County District Attorney's office, told the Houston Chronicle the filing of aggravated sexual assault charges against children so young was "exceedingly unusual."

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Newscore and the Associated Press contributed to this report.