The senators, John Dingell and Bart Stupak, also want the FDA inspectors who were in charge of drug approvals for Ranbaxy to appear before a House of Representatives committee by the end of August.

While Dingell is the chairman of the Committee on Energy and Commerce, Stupak is the chairman of a sub-committee on oversight and investigations.

The duo said last week that FDA's role would be investigated after the US government filed an information-seeking motion in a Maryland court on July 3 alleging that Ranbaxy had fabricated information to gain FDA approval for some of its products.

Confirming the development, the FDA spokesperson said in an e-mail, "We received the letter late last night and logged it in this morning. We will prepare a response."

Reiterating its earlier stand, Ranbaxy said it is not aware of any Congressional Committee inquiry and has not received any communication in this regard.

This could open a pandora's box for India's largest pharmaceutical company as all its products sold in the US may be scrutinised. The US government motion in the Maryland court covered products made at one (Poanta Sahib in Himachal Pradesh) of its several production units.

In a July 22 letter posted on the committee's website, the senators said the motion also indicates that the "FDA was aware of the alleged pattern of systematic fraudulent conduct for at least 18 months, yet did nothing to remove the suspect (Ranbaxy) products from the market..."

Ranbaxy received FDA approval for 141 products, as on December 31, 2007, and another 98 are pending approvals. The US, the largest pharmaceuticals market in the world, accounts for almost a quarter of the company's $1.6 billion annual sales.

In the letter, the senators have said that "FDA's alleged lack of action to remove these suspect products from the market" requires the committee to review the pre-market approval inspections of all currently marketed Ranbaxy drugs.

The FDA has been asked to provide all documents pertaining to pre-approval inspections, findings of those inspections, list of the raw material suppliers and laboratories where Ranbaxy products were tested and the list of FDA personnel who conducted and reviewed each inspection.