The US Justice Department has launched about 100 criminal investigations into major American investors of Swiss banking giant UBS, The New York Times said.
The report said some of the accounts had assets exceeding $100 million, with the biggest valued at over $150 million.
The investigation by federal prosecutors seeks to determine whether the clients had tried to evade taxes by concealing money and assets offshore, the newspaper said.
Under a settlement with the Justice Department, UBS admitted in February to tax fraud by inviting rich US clients to open accounts in Switzerland and thus avoiding declaring their income to the US Internal Revenue Service (IRS). It paid $780 million (euro 571 million) to settle the case.
Citing a person familiar with the matter, the Times said some clients had US and other citizenship and were considered a "flight risk" and were not eligible for an IRS program granting a reduction of the heavy taxes and fines they owe.
Prosecutors were preparing indictments of some UBS clients and could charge them in the coming weeks, the newspaper added.
Many of the UBS accounts "appear to hold less than $10 million in assets", the Times said, noting that level could still translate to jail time of at least two years if clients do not come forward and cooperate.
The US government is currently suing UBS to seek details on 52,000 American clients suspected of tax fraud. The bank in February handed over details on some 300 US clients, but is refusing to hand over information on the other customers.
