The move Thursday by the Commodity Futures Trading Commission, including its unusual announcement of an investigation in progress, comes after crude-oil prices topped $130 a barrel last week and tested all-time highs. On Thursday, light, sweet crude for July delivery settled $4.41, or 3.4%, lower at $126.62 a barrel on the New York Mercantile Exchange.

Lawmakers in Congress have been pressing regulators to crack down on manipulation, as politicians seek to demonstrate ahead of the fall elections that they are responding to soaring gasoline prices.

"It's important that people who are paying high gas prices understand the CFTC is on the case and that we're closely monitoring and in this instance deeply investigating any potential abuse in this important energy market," said Bart Chilton, a CFTC commissioner.

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