Mexico antitrust regulators are looking into seven banks, three of them are U.S.-based, over the possibility of collusion to manipulate bond prices, sources told Bloomberg.

The investigation is focused on local units of JPMorgan Chase (JPM - Get Report) , Bank of America (BAC - Get Report) , Citigroup (C - Get Report) , Banco Santander (SAN) , Banco Bilbao Vizcaya Argentaria SA, HSBC Holdings (HSBC) , and Barclays (BCS) .

These market makers, as they are known, can buy notes from the government and state-owned companies in primary auctions, Bloomberg noted. Some of the banks have already received information requests in relation to the investigation.

Cofece, the Mexican antitrust regulator, first announced its investigation back in April, but did not list the banks it was looking into. The case could cause a headache for the local sovereign and quasi-sovereign debt market that has more than $400 billion of securities outstanding, Bloomberg said.

Investigators are trying to determine if the banks coordinated bids to lower prices and raise yields for holders, costing the government and companies in higher interest rates.

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