While U.S. oil companies blame the global oil market for high gasoline prices, a close analysis of pricing suggests it's not so simple: The run-up at the pump also comes from domestic refining, which is largely controlled by Big Oil.

In consultation with economists, the Associated Press examined pricing trends since 1999. It found evidence that:

\ufffdThe portion of gasoline prices tied to refining has ballooned all on its own, apart from oil.
\ufffdThe suspicion of frustrated drivers is correct: After upward spikes, the price of gasoline drops back more slowly than the price of oil \ufffd and someone pockets the difference.
The petroleum industry knows that many drivers are steamed about both its record prices and profits.

A big chunk of gasoline prices \ufffd almost a fifth \ufffd pays refiners who make gasoline from oil, and refineries have been raising their prices, too.

Charges of refineries can be detected in what's known as their "margin" \ufffd the difference between what they pay for crude oil and what they collect for the gas they refine. Service station costs and taxes add to the final retail price of gasoline.

In a competitive market, when raw material gets more expensive, margins typically shrink, economists say. Not so in the oil business these days. Refiners have somehow managed to fatten their margins through years of rising oil costs.

Since 1999, their average margin has jumped by 85 percent, reaching 43 cents for June, according to AP's analysis of data from the New York Mercantile Exchange. That margin increased by just 20 percent in the seven preceding years.

But refining margins also reflect profit. Some economists and consumer advocates suspect that refiners have intentionally bottled up supply to buoy prices, margins and ultimately profits.

A 2002 congressional study found some evidence it happens, but that doesn't necessarily mean refiners huddled in a back room somewhere, hatching conspiracies. They don't need to. They can each simply decide to crimp output or hoard supply. Such margin goosing is a permissible bid "to maximize their profits," federal investigators said in a 2001 report.

Bob Slaughter, president of the National Petrochemical and Refiners Association, blames high gas prices on high oil prices "which are frankly out of our control" \ufffd not decisions by refiners to hold back on gas. But he also says, "There is no law that says you can make people in an industry invest and expand capacity."

Why wouldn't other refiners simply ramp up their own output and claim a bigger slice of unmet demand?

That has become harder to do, as big refiners have built up market muscle through mergers. The top five now control more than half of U.S. refining capacity, and the top 10 account for three-quarters, according to an AP review of federal data.