The Justice Department sued UBS AG to obtain access to 52,000 accounts belonging to U.S. clients -- some 30,000 more than previously known -- a day after reaching an agreement to settle a criminal investigation that called for the Swiss bank to turn over 250 accounts in a wide-ranging tax-evasion probe.

While the court filing in Miami was expected, the increased number of accounts being sought in a parallel civil inquiry was a surprise to UBS, said a person familiar with the situation.