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WASHINGTON (Reuters) -- The U.S. transportation secretary sought an investigation on Tuesday into a seven-hour airport runway delay that outraged proponents of legislation aimed at forcing airlines to improve customer service.

"While we don't yet have all the facts, this incident as reported is very troubling," Transportation Secretary Ray LaHood said in a statement.

He asked the department's inspector general to look into the Aug. 8 incident involving ExpressJet Holdings Inc., (XJT) a feeder for Continental Airlines Inc. (CAL, Fortune 500) in which passengers were stranded aboard the ExpressJet Airlines regional jet for nearly seven hours overnight.

Flight 2816 from Houston to Minneapolis with 47 passengers was diverted to Rochester, Minn., because of bad weather.

LaHood said he wanted to know if either company "has violated any laws" in the delay.

"We are investigating the incident and will do whatever we can to make sure passengers are not subjected to such situations in the future," LaHood said.

Julie King, a spokeswoman for Continental, said the carrier is working with Transportation Department officials "to respond quickly to their inquiry." Officials of ExpressJet could not immediately be reached. Both companies are based in Houston.

Regulators are considering a rule to address delays in which passengers are required to stay aboard planes holding on the ground for long periods of time.

Legislation moving through Congress, prompted by similar incidents at other airlines in 2007, would require carriers to let people off grounded planes after three hours.

ExpressJet shares finished up a penny to $1.50 on Tuesday on the New York exchange, while Continental shares fell 1.4% to $11.92.

Senator Amy Klobuchar, a member of the Commerce Committee from Minnesota, had pressed for an investigation and vowed to pursue completion of "passenger rights" legislation when Congress returns in September.

"More needs to be done to establish clear processes for the proper treatment of passengers during extended flight delays," Klobuchar said in a letter to executives of Continental and ExpressJet.