WASHINGTON, Nov 15 (Reuters) - The U.S. Federal Energy Regulatory Commission said on Thursday it was investigating two natural gas pipeline companies for possibly overcharging their customers. FERC said Wyoming Interstate Company, owned by El Paso Pipeline Partners, and Viking Gas Transmission Company, owned by ONEOK Partners, may be charging unjust and unreasonable rates for use of their pipelines. The investigation was spurred by cost and revenue data the companies submitted to FERC for 2010 and 2011. "FERC staff's analysis of this information indicates that current rates may allow the companies to recover revenue substantially more than their actual costs of service," the commission said. Both companies now have 75 days to turn over a full cost and revenue study. The Wyoming Interstate Company operates 800 miles of pipeline in Wyoming, Utah and Colorado, while the Viking pipeline system connects four major pipelines serving North Dakota, Minnesota and Wisconsin.