Did a senior UBS banker help American clients evade taxes?

That's what the Justice Department is looking into.  UBS banker Martin Liechti isn't charged with anything -- at least not yet -- but is being held “under a ‘material witness’ warrant” in connection with the DoJ investigation and would remain in the US “pending discussions with the US authorities regarding resolution of his status as a witness”.  UBS is cooperating in the probe.

Martin Liechti, the Zurich-based head of North and South America for UBS’s international wealth management business, was held in connection with a US justice department investigation into investment advice that UBS gave its American private banking clients between 2000 and 2007.

The Swiss lender confirmed on Tuesday that it was also being investigated by the US Securities and Exchange Commission over whether UBS client advisers “engaged in activities in relation to their US-domiciled clients that triggered an obligation for UBS Switzerland to register with the SEC as a broker-dealer and/or investment adviser”.

Top UBS banker detained by US - Financial Times