U.S. Sen. Jon Tester asked congressional investigators Tuesday to examine closed-door road negotiations between the U.S. Forest Service and Plum Creek Timber Co.

The Montana Democrat also asked Agriculture Secretary Ed Schafer to postpone making any changes to Plum Creek's federal road easements until the investigation is complete.

"My hope is just to find out what the heck is going on," Tester said in a telephone interview with Lee Newspapers Tuesday afternoon.

At issue are decades-old agreements Plum Creek has with the Forest Service that allow the timber company to drive across federal land to log its own property.

Since 1999, however, Plum Creek has not been organized as a timber company, but a real estate investment trust. Selling wooded former timber lands for real estate development has since been an increasingly lucrative part of Plum Creek's business, and the company has said it intends to sell more.

For the past two years, Tester said, the company has been negotiating behind closed doors with federal officials to expand the uses of its road easements, which previously dealt only with logging. The proposed new easements would give Plum Creek the right to drive across public land for commercial, industrial or residential development, and according to Tester and several western Montana officials, would open up several tracts of land to real estate development.

With 1.2 million acres -- mostly in Western Montana -- in its possession, Plum Creek is the largest private landowner in Montana.

Tester said Tuesday he first heard of the negotiations two months ago when local officials contacted his office in frustration because they had no ability to weigh in on public lands decisions affecting their counties.

Local officials have said the deal, if it goes through, could put local and state government on the line for providing firefighting and other services to homes in wooded areas previously unimagined for real estate development.

Tester said he was especially concerned the negotiations have been going on in secret for so long.

"Transparency in government is pretty damned important," he said.

Tester said he's been "quizzing" Agriculture Undersecretary Mark Rey -- who oversees the Forest Service -- for two months to get details about the negotiations to no avail.

He said he doesn't know if the negotiations and road changes are necessarily illegal, but enough legal and other questions surround the changes that an impartial investigation is in order.

Tester and Sen. Jeff Bingaman, D-N.M., jointly asked the Government Accountability Office, the investigative arm of Congress, to conduct the probe. In their Tuesday letter to the agency, the senators wrote that as recently as 2006 a Forest Service official in Seeley Lake concluded that such road easements were only for logging and could not be used for other purposes.

But two years later, in 2008, a Forest Service lawyer took exactly the opposite position, they wrote, concluding that such easements were for whatever the timber company wanted to use them for.

Rey has said the negotiations were only to "clarify" the easements and that he would release more information about the changes. But so far, Tester said, the undersecretary has failed to do so.

Tester spoke with Rey Tuesday and said the undersecretary agreed to participate in the investigation. Rey has also assured Tester that he will not make any decision on the roads until giving the senator a presentation about the changes next month.