Hercules officials say human error wasn't to blame for last week's explosion and fireball that damaged a $15 million rocket-fuel mixing building.

"We are confident of one thing at this point," said Ruth L. Novak, Bacchus Works vice president and general manager. "Our people were following the operating procedure, obeying all the rules."Mixing operations for the fuel to power the Delta II space booster rocket motor remain on hold pending completion of the joint investigation by Hercules and the U.S. Department of Defense.

"It will likely be five to 10 more days before we are able to draw any more conclusions," she said in a prepared statement released Tuesday morning.

Novak said the investigation is proceeding slowly and cautiously, in an effort not to destroy any evidence and to protect the safety of investigators and cleanup crews.