It was the latest blow to Mexico's biggest retailer since press reports in April suggested that Wal-Mart de Mexico, known as Walmex, may have been paying middlemen to facilitate store permits with bribes.
Walmex V-series shares fell 6.2 percent to 35.80 pesos ($2.70) a share on Mexico's stock exchange Wednesday following release of the letter from U.S. Reps. Henry Waxman and Elijah Cummings describing the new allegations.
"We have obtained internal company documents, including internal audit reports, from other sources suggesting that Wal-Mart may have had compliance issues relating not only to bribery, but also to 'questionable financial behavior' including tax evasion and money laundering in Mexico," the two U.S. congressmen wrote in the letter to Michael Duke, CEO of Wal-Mart Stores.
Walmex said in a statement that "it has no knowledge that (it) is being investigated by Mexican authorities concerning these issues."
"If any such investigation were to be initiated and Walmex notified of it, Walmex will cooperate with the authorities and to the extent permitted by law, will publicly disclose such facts," the statement said.
The congressmen have launched an investigation into whether Wal-Mart may have violated the Foreign Corrupt Practices Act, which prohibits U.S. firms from paying bribes abroad.
The congressmen told Duke in the letter that "although you have stated on multiple occasions that you intend to cooperate with our investigation, you have failed to provide the documents we requested, and you continue to deny us access to key witnesses."
Following the press reports in April about possible bribes, Mexican prosecutors said they would look into those allegations, but they have not announced any investigation into money laundering or tax issues.
While it is unclear what the new allegations involve, WalMex operates a bank in Mexico that accepts deposits and offers credit and debit cards.
Drug cartels are believed to launder more than $10 billion each year through banks, exchange houses, businesses and purchases of luxury goods in Mexico.
In July, Mexican regulators levied a $28 million fine on the Mexico subsidiary of London-based HSBC bank for failing to prevent money laundering through accounts at the bank.