WASHINGTON - Federal regulators are six months into a wide-ranging investigation of U.S. oil markets, with a focus on possible price manipulation.

The Commodity Futures Trading Commission said yesterday that it started the probe in December and took the unusual step of publicizing it "because of today's unprecedented market conditions."

Crude prices have risen more than 42 percent since early December, even after a decline of more than $4 yesterday to $126.62 a barrel on the New York Mercantile Exchange. Gasoline prices are nearing a national average of $4 a gallon, up from about $3.20 a year ago.

The commission said it was investigating potential abuses in the way crude oil is purchased, shipped, stored and traded nationwide, but did not reveal details. Also yesterday, the agency announced other initiatives designed to increase transparency of U.S. and international energy-futures markets.

For example, the CFTC said it would immediately require monthly reports from institutional investors who manage funds designed to mimic the price of crude oil and other energy futures. The goal, the agency said, is to identify the amount of such index trading and to "ensure that this type of trading activity is not adversely impacting the price discovery process."

The CFTC also said it had reached an agreement with its British counterpart and Intercontinental Exchange Inc.'s Futures Europe to expand surveillance of energy-futures contracts with U.S. delivery points, including the benchmark West Texas Intermediate crude, which trades on the Nymex.

Analysts said the CFTC action would likely have a limited effect on oil prices, which have risen on a combination of factors, including growing demand in China and other developed nations, the falling value of the dollar, geopolitical tensions, and low interest rates, which have fueled a futures buying binge by institutional investors seeking to ride oil's upward momentum.