You can close the book on an allegation which captivated attention in the waning days of the 2012 congressional contest between Republican John Koster and Democrat Suzan DelBene.

Spoiler alert: It wasn’t true.

If you recall, in the heat of their duel, Koster insinuated DelBene — now the congresswoman — had given special treatment to a prominent supporter of Democratic candidates and causes while she served as director of the state Department of Revenue in 2011.

Koster linked her to allegations made in a whistleblower complaint filed by a veteran employee. That worker alleged the unnamed Democratic supporter avoided paying a $2 million use tax on a personal airplane because of political favoritism.

Koster repeatedly said because it happened on DelBene’s watch, she needed to respond to the charge.

“She says millionaires ought to pay their fair share but apparently millionaires got a break,” he said in a television interview.

As Koster pushed the matter on the campaign trail throughout October, he seemed increasingly frustrated by its failure to gain much traction.

“While our liberal media friends are ignoring this major scandal, we are going to continue asking questions until we get some answers,” he wrote on his campaign’s Facebook page Oct. 23.

This week, an investigation found “no reasonable cause to believe an improper governmental action occurred.”

Former state auditor Brian Sonntag launched the probe soon after receiving the whistleblower complaint alleging the tax should have been levied but higher-ups in the department delayed doing so until it became too late for them to legally collect it.

Sonntag’s successor, Troy Kelley, a Democrat, oversaw the completion of the investigation and issued the findings Monday.

“In review of the case notes and the subjects’ e-mail, phone records and statements from witnesses and subjects, we found the Department never made the determination that use tax was owed by the taxpayer in this case,” investigators wrote.

“Our investigation found no hold was placed on the case that prevented an assessment from being issued,” they concluded. “Additionally, we found no evidence that any employee of the Department intentionally delayed the case in order to allow the statutory time bar to expire.”

No one is identified by name in the seven-page report. Instead, agency employees involved are referred to by title. Nowhere is the director mentioned.

Koster, a Snohomish County councilman, was on bereavement leave and unavailable for comment, according to his staff.

DelBene, who described Koster’s claim as “a baseless accusation from a desperate campaign,” declined to comment on the auditor’s findings.

While she said all along she had no involvement in the decision, Delbene may need to address this matter again when she seeks re-election next year.

Some books are worth re-reading.

Political reporter Jerry Cornfield’s blog, The Petri Dish, is at www.heraldnet.com. Contact him at 360-352-8623 or jcornfield@heraldnet.com.