EVERETT — More than $20,000 belonging to Snohomish County Public Utility District ratepayers has turned up missing from a safe kept at the utility’s Paine Field-based operations center.

The theft occurred in late 2010, according to a fraud investigation report state auditors released Tuesday.

“What’s interesting about this case is nobody could figure out who did it,” said Mindy Chambers, a state auditor’s office spokeswoman.

Neither the PUD nor the Everett Police Department could determine who pilfered the cash. Their review included examining security videos and checking records for electronic access to the building, as well as searching the safe for evidence.

The $21,000 constituted the district’s emergency fund. It is tapped to provide cash advances to line crews so they can buy meals during major storms. Roughly 150 workers are eligible for the cash advances.

The safe was under an employee’s desk in an office cubicle. That gave many employees, contractors and others access to the area. Still, only three workers had the safe combination, officials said. There was no direct video surveillance of the area around the safe.

More than 400 people work at the operations center.

“This is kind of a unique situation,” Chambers said. “It’s a lot of money in an unsecured safe.”

Since the theft, the PUD has made changes to avoid a similar loss in the future.

The utility bought a new safe with an electronic keypad. That allows them to change the combination as needed. Only two people at the PUD will be told the combination.

The PUD also has moved the safe to a senior manager’s office, which is locked when he’s gone. Employees must use identification badges to get access to the area.

The stolen money was kept in $20 bills in a metal box within the safe, PUD assistant general manager of finance Glenn McPherson said.

The safe also contained credit cards, which were not taken, McPherson said.

The theft was discovered Nov. 16, 2010 as a storm approached. McPherson said there was a lengthy internal investigation to try to determine who took the money.

“They looked at unusual access requests to try to see if we could isolate an individual,” he said.

Investigators found employees had legitimate reasons that could be documented when they entered the building in off hours, he said.

Insurance didn’t cover the lost money.

“It was below our deductible,” McPherson said.

The PUD also received a clean audit for its day-to-day operations in a separate report released Thursday.

The district’s electric system serves about 320,000 customers in a service area of 2,200 square miles in Snohomish County and Camano Island in Island County. The district had operating revenue of about $573 million in 2010. Eric Stevick: 425-339-3446, stevick@heraldnet.com.