As part of a widening probe, the U.S. has charged a former UBS AG banker and a Liechtenstein consultant with helping clients avoid taxes by opening secret bank accounts, destroying documents, using Swiss credit cards and filing false tax returns. One client was billionaire California real-estate developer Igor Olenicoff. Mr. Olenicoff set up a web of secret bank accounts in Switzerland and Liechtenstein to avoid taxes on $200 million in assets, a person familiar with the U.S. case said. Mr. Olenicoff has been cooperating with investigators in the wake of his December guilty plea to a criminal count of filing...