America's Commodity Futures trading Commission charged an oil-trading firm, two of its affiliates and two traders with attempting to manipulate oil prices in 2008.

But people familiar with the agency's operations say its concerns stem from tips from sources in the oil-trading world about big market moves that occurred unexpectedly.

One executive involved in oil trading said brokerages have reduced or canceled lines of credit to traders, even telling customers they need to double the amount of margin required from last week.