The other 25 percent of the price increase stemmed from lower gasoline production as refiners moved to using ethanol as the main clean-burning fuel additive and lingering damage from hurricanes Katrina and Rita that reduced refining capacity.
"Our targeted examination of major refinery outages revealed no evidence that refiners conspired to restrict supply or otherwise violated antitrust laws," the FTC said. "We therefore conclude that further investigation of the nationwide 2006 gasoline price spike is not warranted at this time."
Many lawmakers at the time had accused oil companies, which were raking in billions of dollars in record profits, of overcharging U.S. consumers at the pump.
President George W. Bush directed the FTC and the Justice and Energy Departments to look into whether manipulation or other illegal activity by oil companies was behind the sharp rise in gasoline prices.
The national retail monthly average gasoline price jumped from $2.28 a gallon in February 2006 to $2.89 by the beginning of May, and then declined slightly through June. Prices started rising again in July and hit a peak of $3.02 a gallon during the second week of August, and then took a steep decline to $2.18 by the end of October.
The FTC said its investigation found the increases in motor fuel prices "were caused by a confluence of factors reflecting the normal operation of the market."
FTC Commissioner Jon Leibowitz dissented from the report's conclusions and issued a separate statement that said the agency developed a "theoretical model" for why gasoline prices likely increased.
Leibowitz said he believes "there was profiteering (by oil companies) at the expense of consumers."