LIBERTY, MO. ? Federal investigators said it could be more than a year before they determine why a school bus went out of control Monday, injuring 23 students and killing two men who were waiting in their cars at a stoplight.

Three students checked out of Kansas City’s Children’s Mercy Hospital on Wednesday and two more were scheduled to leave, said hospital spokeswoman Jennifer Benz.

One of the students still hospitalized, Andrew Hubbard, was in critical condition. The family of a second hospitalized student asked that the child’s condition not be released.

The bus driver, 45-year-old Irma Denise Thomas, remained in stable condition Wednesday at Liberty Hospital. All other students hospitalized there have been released, the hospital said.

Thomas, who has been driving school buses for the school district for seven years, had a clean driving record and no traffic citations, according to the State Department of Revenue.

Supt. Scott Taveau said Thomas told him after the crash that she could not get the bus to stop, though she didn’t say whether the brakes had failed. It was her second bus route of the day, school officials said. She had transported about 30 secondary-level students on a swing that ended at 7:20 a.m. — an hour before the crash.

The Federal Transportation and Safety Board likely will not release the findings from its investigations for a year to 18 months, chief investigator Gary Van Etten said Wednesday.

“We want to have good accurate information no matter how long it takes,” Van Etten said.

David M. Gleason, 53, a lawyer from northern Kansas City, and David Sandweiss, 49, an advertising agency president from Liberty, were killed as their vehicles sat side by side waiting for the traffic light to change.

Services for Sandweiss will be at 2 p.m. today at Park Lawn Northland Chapel. A memorial service for Gleason will be at his home at 10 a.m. Friday.

A videotape camera on the bus was not working, said Van Etten, who is leading the seven-member NTSB team reviewing the crash. He said investigators still had information from the global satellite positioning system inside the bus, which recorded the location and speed of the bus every three minutes.

Taveau said school records showed the videotape camera last recorded April 26.

Liberty Police Lt. Mark Misenhelter said investigators had interviewed at least 20 eyewitnesses to the accident. They also spoke with children who were on the bus, and they hope to interview more.