HERMISTON -- Officials at the Umatilla Chemical Depot in Hermiston are stumped so far in determining what caused two recent accidents in which chemical weapons rockets caught fire as they were being destroyed.

The latest fire on Saturday was the third time a rocket has caught fire at the depot since incineration operations began in September. While the cause of the first fire was determined, investigators don't know what caused the last two incidents, both of which were this month, said Mary Binder, depot spokeswoman.

The first incident occurred in November. Depot officials believe it was caused when rubber tubing that sprays water to cool the blade as it cuts the rockets was deflected slightly, causing the blade to overheat and start the fire.

The last two fires, however, occurred in separate areas. Those happened as the rocket shearing machine was making the fifth of eight cuts through the rocket's motor, which contains a compressed propellant, a mixture of nitroglycerin and nitrocellulose.

Dennis Murphey, administrator of the Oregon Department of Environmental Quality's Chemical Demilitarization Program, said investigators are comparing the lot numbers of the propellant to see if there were similarities.

"Anniston had one incident that appears to be similar, and the lot number was close," said Murphey, referring to Anniston, Ala., where the Army has an incinerator site similar to Hermiston's.

Murphey said the Army and Washington Group International, which runs the incinerator about 35 miles south of the Tri-Cities, are investigating the incidents.

"We process 11,000 rockets without it happening, then processed about 2,000 more and it happened again," he said.

The 8-foot M55 rockets are drained of GB sarin nerve agent before the rockets themselves are cut up and burned. Umatilla has the most M55 rockets of the eight U.S. chemical weapons storage sites.

Binder said there were no injuries during Saturday's incident but equipment damage is still being assessed. The fire started at 1:30 p.m. and lasted 10 minutes, she said. Air was automatically shut off to the explosive containment room and the fire burned itself out.

"Preliminarily, it looks like there is not as much damage as the last time," Binder said.

Binder said the plant's two explosive containment rooms are specifically built to handle potential explosions and are sealed to prevent chemical agent from escaping.

Each room also has cameras that allow incinerator operators to see what's happening.

"We showed the video of the last rocket fire at the Citizens Advisory Committee meeting last week," Binder said.

The last fire shut down incineration operations for eight days. Binder said she didn't know when burning will begin again following the most recent incident.

Nationwide, there have only been about a half-dozen chemical weapon rocket fires since incineration began at Johnston Atoll in the early 1990s, Binder said. That means the Umatilla Chemical Depot now accounts for half of the total.

It is expected to take about 10 years to destroy the Umatilla depot's stockpile of 220,604 munitions and containers, which are filled with 7.4 million pounds of deadly nerve and mustard agents.