(AP) — Propane prices in the Midwest have soared in recent days, leading consumers, retail suppliers and political leaders to ask whether there is more at play than supply and demand.

The spike has been blamed on a supply problem caused by farmers using propane-fueled equipment last fall to dry wet grain, as well as a bitterly cold winter and the temporary shutdown of a major supply pipeline in December.

But Iowa Sen. Chuck Grassley has asked the Federal Trade Commission for more oversight of the propane market “to prevent possible anti-competitive behavior or illegal manipulation.”

Terry Davis, owner of Country Propane in Milo, Iowa, says his average residential customer would have paid about $800 to fill up a 500-gallon tank two weeks ago. On Friday, the cost soared to about $2,000.