Are high oil prices due to hedge fund speculation?

The economist Paul Krugman advances an interesting argument today in the New York Times against the idea that high oil prices are due to hedge fund speculation.

He believes that speculative buying can lead to persistent high prices (which has been the case for the last few years) only if there is physical hoarding. Yet oil inventory level has been normal for this period.

Indeed, I have been trying to find a mean-reverting strategy to trade oil and oil-related assets for some time now. So far, none have outperformed (even on a risk-adjusted basis) just buy-and-hold energy stocks for the long term!