11 Injured as Artillery Shell Explodes : Munitions: The arm of a woman employee at the Chino Hills ordnance plant is severed in the blast. Investigation is under way.

CHINO —  An Ontario woman was hospitalized in critical condition and at least 10 other workers were treated for lesser injuries after an artillery shell exploded Tuesday afternoon on an assembly line at the Aerojet Ordnance plant in Chino Hills, local officials said.

Otilia Belmontes, 36, was airlifted to the San Bernardino Medical Center where she was undergoing emergency treatment in the hospital’s burn ward. Her left arm was severed above the elbow by the force of the blast, according to Battalion Chief Bruce Clark of the Chino Valley Fire Department. Clark said the woman also suffered massive trauma to her right arm, facial cuts and powder burns.

An earlier report indicated that two workers were critically hurt. However, that information later proved to be inaccurate.

Some of the injured workers were taken to Chino Community Hospital, Ontario Community Hospital and Doctors Hospital in Montclair. Most of those workers suffered shrapnel and percussion wounds, Clark said.

Eleven other workers who were examined at the scene did not require hospitalization.

Company and fire officials declined to identify injured workers before their relatives were notified.

Edie Cartwright, a spokeswoman for the company, which manufactures artillery rounds for the military, said the accident occurred at 1:05 p.m. on a machine that is part of the “load and assembly” process at the facility in the remote Los Serranos area of Chino Hills, about 30 miles east of Los Angeles.

Fire Department spokesman Kurt Mantwell said the explosion apparently was caused by a malfunction of a loading rack on which primers--detonating devices--are installed in the shells. He said the blast tore a 4-by-8-foot hole in the ceiling of the room where the shells were being assembled.

“It was over in a flash,” said Lt. Larry Swope of the San Bernardino County sheriff’s office in Chino. “There was no fire.”

Reporters were not allowed on company property after the blast. The building was described as a reinforced concrete bunker carved into a hillside.

Cartwright said the loading rack handles ammunition ranging from 8 to 12 inches in diameter. Mantwell said the shells being handled Tuesday were smaller.

Company security personnel and experts from the California Occupational, Safety and Health Administration began an investigation to pinpoint the cause of the accident.

Cartwright said the plant at 2700 Soquel Canyon Road employs about 150 people.

Joe Velasquez, who worked at a companion Aerojet plant in Norwalk, said the Chino Hills facility assembles artillery rounds from powder, shell casings, primers and warheads manufactured elsewhere. He said some of the shells contain high explosives; others are incendiary rounds designed to burn on contact.

Officials did not say which type of shell was involved in the accident.

Fire officials said the plant has a good safety record and described Tuesday’s blast as only the second major incident at the plant in 13 years. A 1978 flash fire at the plant injured six workers.

Ward reported from Chino Hills, Malnic reported from Los Angeles.