LOS ANGELES (AP) _ A day after a problem was found with the hydraulic system on a garbage truck, the steel arm of its trash compactor shot out the side and ripped into a school bus it was passing, killing two boys.

Another boy was critically injured, but the other elementary students on the packed school bus escaped with minor injuries.

The 12-foot hydraulic arm, which pushes a ram to compact trash inside the truck, broke through the side of the vehicle Wednesday with a force of 1,500 pounds per square inch, said Roland Silva, spokesman for the Sanitation Department.

The arm plowed into the driver's side of the bus, slicing through the windows and bending the metal frames. ``It was like a missile coming out of a launch pad,'' Silva said.

The children who died were hit in the head, emergency workers said.

Children were screaming as customers ran from a nearby gas station to help.

``I heard a lot of screams and crying,'' said Paul Gulrajani, who works at the station. ``The kids looked scared, frightened. It was a very emotional scene.''

City officials were investigating why the garbage truck was in use when a driver had reported a problem with the hydraulic system only a day earlier.

``He heard a noise as he was unloading the vehicle at the landfill,'' city sanitation spokeswoman Gyl Elliott said.

Mayor Richard Riordan immediately ordered inspections of the city's 390 trucks with a similar design, about half the sanitation fleet.

Francisco Mata and Brian Serrano, 8-year-old third-graders at Glen Alta Elementary School, were killed in the accident, said Ria Parody, spokeswoman for the Los Angeles Unified School District.

Francisco Mata Sr., said his son, one of four brothers, dreamed of becoming a police officer or a firefighter.

``My son was the No. 1 student in his class,'' Mata said. ``He loved to study. He loved to read.''

``What can I do? It was a freak accident,'' he added. ``I don't blame the driver of the bus or the driver of the truck.''

Eight-year-old Mario Garay, Francisco's cousin, was in critical condition today with a skull fracture, said Ron Yukelson, spokesman for Childrens Hospital of Los Angeles.

Bus driver Humberto Ojeda and truck driver Kenneth Fox, 28, were visibly distraught after the collision. Witnesses said Fox retracted the hydraulic arms, pulled his truck to the side of the street, then ran to the bus.

The children, 48 in all, were being transported to Glen Alta Elementary School. Psychologists have been made available to counsel the students who were aboard the bus.

Mourners turned the accident site into a makeshift memorial Wednesday night, leaving burning candles.