WASHINGTON?While the industry often blames gasoline price spikes on market-driven shortages, a congressional investigation has found that some oil companies reduce supplies when markets are tight to force up prices and profits.

The investigation by a Senate subcommittee found that industry manipulation of gasoline supplies exacerbated tight fuel markets and helped produce some of the sharp price spikes over the last three years, especially in the Midwest.

“In a number of instances, refiners have sought to increase prices by reducing supplies,” says the 396-page report released Monday by Sen. Carl Levin, chairman of the Senate Permanent Investigations Subcommittee.

Levin, in a statement, said that the subcommittee investigation “confirms what a lot of us have been saying for some time?that when it comes to gasoline, there is too little competition and too much concentration in many markets.”