Jim Clark, who was the former governor's chief of staff, agreed to plead guilty to a single count of conspiracy fraud in a filing in U.S. District Court in Anchorage. He was scheduled to enter his plea at an arraignment hearing on Tuesday.

Clark admitted to taking $68,550 in illegal contributions from the state's largest oil-services company, VECO Corp, for Murkowski's failed 2006 reelection bid in exchange for working on VECO's behalf to secure an industry-friendly version of tax legislation, according to the plea agreement.

He is the first official from the Murkowski administration to be charged in a federal criminal investigation that has so far resulted in convictions of three former state lawmakers, the indictment of a fourth and guilty pleas from two top VECO executives and one former lobbyist.

Murkowski, who was also a former U.S. senator, was soundly defeated in the 2006 Republican primary by Sarah Palin, Alaska's current governor who ran as an anti-corruption reformer.

Clark and VECO conspired to hide the illegal contributions "in a manner so that the public would be deceived and the payments would not be disclosed, as required by law," according to charging documents.

The federal investigation centers around a revision of an oil-tax law that passed the state legislature in 2006 at Murkowski's urging. Bill Allen and Rick Smith, two former VECO executives, pleaded guilty to bribing state lawmakers for a pro-industry version of the bill and other favorable actions.

Former state Senate President Ben Stevens, son of powerful U.S. Senator Ted Stevens, received much of that bribe money, Allen and Smith testified in court last year.