The U.S. Department of Agriculture’s inspector general should investigate a pending agreement between the Forest Service and Plum Creek Timber Co., Sen. Jon Tester, D-Mont., said Friday.

Tester sent the inspector general a letter requesting the investigation into the agreement, negotiated privately, that would amend part of a long-standing deal for Seattle-based Plum Creek’s use of Forest Service roads in areas where the company owns timberlands.

A call seeking comment by Agriculture Undersecretary Mark Rey, who oversees the Forest Service, was not immediately returned by his office Friday.

A preliminary investigation by the Government Accountability Office ‘‘raised serious questions’’ about the Forest Service interpretation of road easements, Tester and Sen. Jeff Bingaman, D-N.M., wrote in the letter. Bingaman is chairman of the Senate Committee on Energy and Natural Resources.

They expressed concern that the amendment ‘‘could lead to major land-use changes with major environmental impacts on the national forests, may put public safety at risk and could increase costs for the government and its taxpayers.’’

Missoula County commissioners have said that amending the agreement on Plum Creek’s use of Forest Service roads could foster real estate development on company timberlands. Housing development could require local governments to provide public services, such as fire protection, in remote places and at high cost, the commissioners say.

Plum Creek and Rey have said the company has long had use of Forest Service roads in Montana to access corporate timberlands for any legal purpose. County officials find that conclusion overly broad.

Plum Creek CEO Rick Holley said last summer that amending the agreement would not change purposes for which the roads may be used. Rather, he said, provisions would be added, such as one requiring future buyers of Plum Creek lands to reduce wildfire risk on their property.

Rey said earlier this month that he wants the amended agreement to become final before he leaves office with the change in presidents.

‘‘If this a good agreement, there should be no concern about getting this (adoption) done rapidly,’’ Tester said in a telephone interview Friday.

A sound deal should withstand the change in White House administrations, Tester said, and taking the time to further examine both the amendment and the circumstances of its negotiation should not pose a problem even if that delays putting it on the books.