A worker accidentally tested a live nerve gas monitor instead of a machine that had been sidelined for maintenance, officials said Monday, causing the positive sarin reading that disrupted work at the laboratory of the Anniston chemical weapons incinerator last week.

Friday morning, workers were evacuated from the laboratory building at the incinerator after an alarm sounded, indicating sarin had been detected in the air. A later test confirmed the presence of sarin in the backup tube designed to foolproof the detection process.

Investigators suspected a mistake by an employee because both the initial detection and the backup test found identical levels of the lethal chemical, said Donavan Mager, a spokesman for Westinghouse Anniston, the contractor operating the incinerator.

Slight variations in the sarin levels are inevitable in two readings under normal conditions, Mager said. "So they knew then something wasn't right," he said.

Further, both tests showed identical contamination with alcohol, he said. Lab technicians routinely test and calibrate monitors in the laboratory by injecting them with sarin diluted in water and alcohol.

However, all of the lab workers denied making such an error until Monday morning, Mager said.

According to a statement from the Army, "management is looking at the appropriate disciplinary actions for the employee who was involved."

"We realize employees make mistakes," Westinghouse project manager Bob Love said in a printed statement released Monday. "The problem lies in that the employee did not admit to the mistake when immediately questioned and continued to deny it until this morning."

The monitor that set off an alarm Friday sits at chest level on a stand in the same room as equipment intended for maintenance, Mager said.

The incinerator has continued to operate throughout the weekend. Three months ago, the incinerator began its mission to destroy more than 660,000 chemical weapons that have been stockpiled at the depot since the beginning of the Cold War.