(Recasts; Adds quotes from lawmakers, details, background on law enforcement investigation)

WASHINGTON, July 22 (Reuters) - Two leading U.S. Democrats said on Tuesday they are investigating whether Food and Drug Administration knowingly allowed the sale of Ranbaxy <RANB.BO> drugs that may have been backed by fraudulent data.

The congressional investigation stems from information released as part of a probe by U.S. law enforcement authorities into whether the Indian drugmaker submitted false data to support generic drug applications and tried to hide manufacturing violations, the lawmakers said.

Reps. John Dingell and Bart Stupak said in a statement they want to know if the FDA "knowingly allowed drugs suspected of being fraudulently approved and manufactured in violation of Good Manufacturing Practices to continue being sold by Ranbaxy Inc in the United States."

Ranbaxy Laboratories Ltd, India's largest drugmaker by sales, has agreed to a $4.6 billion takeover offer from Japan's Daiichi Sankyo <4568.T>.

The lawmakers pointed to a legal motion filed July 3 by the Justice Department and the U.S. Attorney's Office on the FDA's behalf.

The motion, which was seeking to enforce subpoenas for documents, stated that the FDA was aware of allegations of fraudulent conduct by Ranbaxy for at least 18 months but did nothing to remove suspect products from the market, the lawmakers said.

"If true, these statements would call into serious question whether the leadership of the agency ... have met even the minimum requirements of due diligence," the lawmakers said.

FDA spokesman Christopher Kelly said the agency had received the lawmakers' letter and would respond directly to them.

Ranbaxy has denied allegations that it sold misbranded or adulterated drugs and said it was cooperating with the three-year-old probe by U.S. authorities.

Dingell chairs the House of Representatives Energy and Commerce Committee and Stupak heads its investigations subcommittee. Both are from Michigan.

The lawmakers, in a letter to the FDA, asked for documents relating to its inspections of Ranbaxy facilities and its suppliers, plus other information. (Reporting by Lisa Richwine, editing by Gary Hill and Braden Reddall)

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