WASHINGTON (AP) _ The Federal Trade Commission has opened a formal investigation into soaring gasoline prices in some areas of the Midwest and will begin issuing subpoenas to oil companies by the end of the week, congressional sources said Tuesday.

FTC Chairman Robert Pitofsky told some members of the Illinois congressional delegation that the sudden price spikes in the Chicago and Milwaukee areas and elsewhere are ``sufficiently questionable'' to warrant a formal investigation into possible price gouging and collusion, according to one lawmaker present.

``The fact that they're moving forward with this investigation will be a clear signal to the oil companies to bring down prices immediately,'' said Sen. Richard Durbin, D-Ill., who was among those who met with Pitofsky on Capitol Hill.