US media said the investigation began before UBS proposed to take over Credit Suisse, which was on the verge of collapsing.
The Justice Department also reportedly issued subpoenas to employees of several major US banks. Insiders have stated that the department is focused on uncovering which bank employees dealt with sanctioned clients and how these clients were vetted over the last several years.
The Justice Department may deepen its investigation if it finds the bankers broke any laws.
Credit Suisse declined to comment on the matter and UBS did not respond to a request for comment, the report said.