The US energy regulator has taken civil action against a Dutch-based oil trading firm and three top employees for manipulating crude oil prices.
The US Commodity Futures Trading Commission complaint relates to Optiver Holding and two subsidiaries.
As the price of oil has risen sharply since last year, the watchdog has turned its focus on the activities of speculators.
It is feared that traders have contributed to swings in oil prices.
Bastiaan van Kempen, the boss of Chicago-based Optiver, a subsidiary of Optiver Holding, was named as a defendant along with trader Christopher Dowson and Randal Meijer, a supervisor of Optiver.
The CFTC alleges that they attempted to manipulate the price of different types of oil during 11 days in March 2007.
Out of 19 attempts, five were successful earning the accused about $1m (£502,000).
Serious step
"These charges go to the heart of the CFTC's core mission of detecting and rooting out illegal manipulation of the markets," said CFTC acting chairman Walt Lukken.
In a statement, Optiver Holding said: "We have learned that the US Commodity Futures Trading Commission has filed a civil lawsuit against Optiver.
"We have received a copy of the complaint and are reviewing it. We take the CFTC's action very seriously and are treating it with the utmost attention and care."
The lawsuit comes as US lawmakers review a plan put forward by the CFTC to prevent speculators trading on the London oil market to escape strict US rules on trading.
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