Former Interior Secretary Gale Norton says the Obama administration "wasted millions of taxpayer dollars" in a now closed probe of her ties to an oil company where she took a job after leaving government.

Norton told The Associated Press that the nearly two-year investigation by the Interior Department's inspector general was "an attempt to find imagined wrongdoing."

Norton, who served as Interior secretary from 2001 to 2006, was accused of using her position to steer lucrative oil leases to Royal Dutch Shell PLC, where she took a job nine months after leaving Interior.

Mary Kendall, the Interior Department's acting inspector general, said Friday that that the investigation failed to prove a conflict of interest. Kendall said the Justice Department has closed a criminal probe in the case.

The Interior investigation focused on a 2006 decision by the department's Bureau of Land Management to award three oil shale leases on federal land in Colorado to a Shell subsidiary. Oil from the leases could eventually earn the company hundreds of billions of dollars.

Oil shale is a still-developing energy source that involves the extraction of oil from sedimentary rock. In the United States the largest deposits appear in three Western states — Colorado, Utah and Wyoming — with the vast majority on federal land.

Investigators examined whether Norton, President George W. Bush's first Interior secretary, violated a law that bars federal employees from discussing employment with a company if they are involved in a decision that could benefit that company. Months after granting the leases to Shell, Norton left the agency.

Shell hired her later that year as an in-house counsel for its unconventional fuels division, which includes oil shale. She no longer works for Shell, the company said.

In a statement provided to AP, Norton said she followed federal rules on employment. She did not talk to Shell or any other potential employer until after she left the government and was unemployed for nine months, Norton said.

She called oil shale a "vast resource," adding that she was so enthusiastic about it that she spent more than three years working on Shell's research efforts during her time at the company. "Even though I am no longer with Shell, I wish them well in their efforts. I hope the Obama administration will stop creating obstacles to this important part of America's energy future," Norton said.

A spokesman for the Interior inspector general said the office stands by its report, which found that the land management bureau appeared to give Shell preferential treatment in at least two instances. Those events could not be linked to Norton, the report said.

"We appropriately and thoroughly investigated serious allegations," said Kris Kolesnik, associate inspector general for external affairs.

He declined to comment on Norton's complaint about the investigation's cost.