Authorities on Monday arrested an obscure, now-former King County official in a long-running corruption investigation.

Allan "Benny" Thomas, who served as the longtime commissioner of King County Drainage District No. 5, was arrested along with his wife, Joann Thomas, on a mail fraud charge, federal prosecutors said. The FBI said the couple cheated the district's taxpayers of about $470,000 from 2012 to this year.

A KING investigation last spring revealed that money supposedly being used to maintain 18 miles (29 kilometers) of storm water drainage trenches around the Enumclaw area instead went to a company controlled by the Thomases. It was spent on personal expenses like a home loan and hay and equipment for their dairy farm, the FBI complaint said.

Thomas resigned in May. He was a commissioner for more than 35 years, though no election had been held since 1986.

The couple was due in court Monday afternoon. Their attorneys, John Henry Browne and Emma Scanlan, did not immediately return a call seeking comment.

Drainage districts are areas where landowners are taxed to pay for ditch maintenance to help prevent flooding. Each fall, a district's three commissioners estimate how much money they need to for the work and then alert the county, which assesses that amount in taxes from the landowners.

When work is performed, the district alerts the county, which sends checks so the district can pay its contractors. There are about 600 landowners in King County's District 5.

Enumclaw officials began investigating the drainage district in late 2017 amid questions about whether poor or unperformed maintenance was to blame for flooding in the area and forwarded what they found to the Washington State Auditor's Office. The auditor determined in a report released in May that the contractor typically used by the district, AC Services, was controlled by the Thomases.

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Banking records show that deposits to AC Services' account were quickly transferred to the Thomases' personal bank account, the FBI complaint said, and in 2018, after the couple learned that Enumclaw officials were investigating, they began sending the payments to another company they controlled, City Biz.

The couple initially had Allan Thomas' son from a previous marriage, Alexander Thomas, set up AC Services, and he told investigators he had not performed any work for the drainage district since doing two jobs in approximately 2012. The FBI's complaint said it appeared that his name had been forged on checks deposited by AC Services.

In an interview with the state auditor's office, the Thomases claimed the payments were legitimate. But another commissioner, Kennet Olson, contradicted that in interviews with the auditors and the FBI.

Olson told the FBI that Thomas appointed him about seven years ago, and that Thomas claimed no elections were needed because no one else wanted to serve, the complaint said. Olson, who was not charged, said he never dealt with the district's finances; had never seen or approved the invoices sent by AC Services; and had not seen any ditch maintenance being performed since some work that was done on his property in 2012.

Other residents interviewed by the FBI were similarly at a loss, the complaint said.

"Most had not seen any such work, and many noted that they had observed regular flooding of fields after rains," the complaint said. "One resident did say that he had seen ALLAN B. THOMAS working on a ditch or ditches 'years ago.' Another said that he had observed that work had been done on a portion of a ditch that was located on the THOMASES' farm, and that the THOMASES' farm did have equipment (such as a backhoe and a dump truck) that could be used to work on drainage ditches."

The King County Council has appointed three new commissioners for the district.