Among the Dutch bank's 119 branches in California, the tiny outpost along Mexico's border needed weekly armored truck visits -- sometimes more than one -- to haul away all the U.S. dollars being deposited.

The deluge, described by a person familiar with the bank's operations who said it picked up after 2010, came from businesses just across the border in Mexicali. While other Rabobank branches needed currency to distribute to customers, Calexico was shipping it out.

The armored car trips, which persisted for years, offer a clue about what prosecutors may find in a widening investigation into whether Rabobank Groep was vigilant enough against money laundering. Rabobank shut the Calexico branch in January under a cloud as federal investigators sought to determine if the bank had ignored signs that its California operations may have been used by drug cartels to launder funds.