A city investigation published Thursday found a yellow bus carrying six nonverbal autistic students in Brooklyn was lost for more than four hours on the way to school last year while a panicked group of parents and school officials struggled to locate the children.

The report by the Special Commissioner of Investigation, a watchdog agency for the city’s public schools, said a bus operated by Consolidated Bus Transit was scheduled to arrive at a Brooklyn elementary school before 9 a.m. on Dec. 16, but did not show up until 1:25 p.m. that day.

Parents tried to track the bus’s winding route via AirTags placed in their children’s backpacks while school staff struggled to reach the bus company, the investigation found. When the bus of 6- to 8-year-olds did arrive, one student was lying in a pool of urine, according to the report.

It’s the latest in a string of incidents that have parents and regulators calling for urgent reforms to the city’s dysfunctional school bus system. While the majority of New York’s students walk, drive or take public transit to school, about 155,000 rely on yellow buses. Half of those using the buses have disabilities.

Parents have long complained of delays, circuitous routes, broken GPS systems and unresponsive call center staff. They said improving the bus system will require passing legislation in Albany so the city can rebid and replace 45-year-old bus contracts with new contracts that add strict requirements for vendors and strengthen service for students.

“It is highly disturbing that six nonverbal children were left on a school bus for hours while their whereabouts remained unknown to school officials,” Special Commissioner Anastasia Coleman said in a statement. “These particularly vulnerable students rely on the adults entrusted with their care to ensure their safety.”

According to the report, the bus driver said he stopped at two incorrect addresses before the bus company called with a correct address for P.S. 289 in Crown Heights. The driver and attendant were both fill-ins temporarily assigned to the route, and the bus’s GPS system was “inoperable,” the report said. During the four hours when the bus was missing, worried school staff filed a police report.

Representatives for the bus company did not immediately respond to requests for comment.

Education department officials said the bus driver and attendant were immediately suspended. The special commissioner of investigation’s office recommended that the two employees never be allowed to supervise city public school children going forward.

At the start of the school year, thousands of students who needed buses to get to school did not have access to them . Education department officials said an ongoing driver shortage has been exacerbated by an increase in the number of children with disabilities who need busing.

Mayor Zohran Mamdani has said his administration is working to address both the immediate and structural problems with the bus system.

“We are doing everything we can to ensure that every single student is able to get to school,” Mamdani said at a press conference on the first day of school. “And we are also working at addressing this long-term issue.”