As oil prices top the $40 per barrel mark, the Commodity Futures Trading Commission (CFTC) is said to be launching a probe into the workings of the largest oil exchange traded fund (ETF).

The issue with the United States Oil Fund (USO), reports Matthew Leising for Bloomberg, is whether the fund and other investors affected the price of oil on Feb. 6. The probe is part of a larger oil-market investigation that the CFTC has been conducting since last year.

In discussions with industry sources, it’s suggested that perhaps the actual subject of the CFTC’s interest is the trading done by others that may have affected end-of-day pricing, instead of within the oil fund itself.