Credit Suisse and UBS are among the banks under scrutiny in a US Justice Department probe into whether financial professionals helped Russian oligarchs evade sanctions, according to people familiar with the matter.

The Swiss banks were included in a recent wave of subpoenas sent out by the US government, the people said.

The information requests were sent before the crisis that engulfed Credit Suisse and resulted in UBS's proposed takeover of its rival.

Subpoenas also went to employees of some major US banks, said two people with knowledge of the inquiries.

The Justice Department inquiries are focused on identifying which bank employees dealt with sanctioned clients and how those clients were vetted over the past several years, according to one of the people.

Those bankers and advisers may then be subject to further investigation to determine if they broke any laws.

Credit Suisse declined to comment and UBS did not respond to a request for comment.

Before the Russian invasion of Ukraine resulted in expanded sanctions, Credit Suisse was well-known for catering to wealthy Russians.

At its peak, the bank managed more than $60 billion for Russian clients, who generated between $500-600m a year in revenue for Credit Suisse.

At the time it discontinued its business with individual Russian clients last May, Credit Suisse held about $33 billion for them, 50% more than UBS, despite the latter's larger wealth management business.

The Justice Department last year launched its KleptoCapture task force to enforce sanctions on wealthy Russians who are political allies of President Vladimir Putin.

The US government has since seized a number of yachts, private planes and luxury properties.

Banks can face serious penalties for violating US sanctions.