WASHINGTON — The State Department’s inspector general will conduct a special investigation of the handling of the pending decision on the proposed Keystone XL pipeline in response to reports of improper pressure on policy makers and possible conflicts of interest, according to documents released on Monday.

Harold W. Geisel, the senior official in the inspector general’s office, told top agency officials in a memorandum dated Friday that he would open the review “to determine to what extent the department and all other parties involved complied with federal laws and regulations” relating to the pipeline permit process.

The internal investigation could delay the Obama administration’s decision on whether to approve the $7 billion project, which would carry oil extracted from Canadian tar sands to refineries in Oklahoma and along the Gulf Coast. The State Department had set a deadline of year’s end to determine whether the pipeline is in the national interest, but officials suggested last week that the schedule could slip.

Objections by states along the pipeline right of way — particularly Nebraska, which is asking for a review of the proposed route — could also delay the decision for months.