US federal prosecutors subpoenaed several banks last month as part of a criminal investigation into possible manipulation of the US Treasuries market, Bloomberg reported on Monday.
The banks include UBS Group AG, BNP Paribas SA and Royal Bank of Scotland Plc, Bloomberg reported, citing people familiar with the matter.
A series of class action lawsuits have accused various banks and brokerages of conspiring to manipulate US Treasury auctions.
The lawsuits have alleged that the banks colluded to manipulate Treasury Department auctions and the pricing of Treasury securities, as well as derivative products such as futures, whose value is pegged to the Treasury.
UBS has flagged a probe related to US Treasury securities in its earnings reports.
"UBS and reportedly other banks are responding to investigations and requests for information from various authorities regarding US Treasury securities and other government bond trading practices," the Swiss bank said in its latest quarterly report last month.
UBS, BNP Paribas and the US Justice Department all declined to comment when contacted by Reuters. RBS did not immediately respond to a request for comment.