For years, Citigroup employees feared that millions of dollars the bank was moving to Mexico might be suspicious. Yet in many cases, the bank did not alert regulators or step up its monitoring for money laundering, federal prosecutors said Monday.

Even as the Citigroup unit Banamex USA was growing to dominate remittances from the United States to Mexico, the bank did not properly safeguard its systems from being infiltrated by drug money and other illicit funds, prosecutors said.

On Monday, Citigroup agreed to pay $97.4 million in a settlement after a long federal investigation into Banamex USA. In exchange, the Justice Department will not file criminal charges against the bank in connection with inadequate oversight of Banamex USA, which is based in California.

As part of the agreement, Banamex USA “admitted to criminal violations by willfully failing to maintain an effective anti-money-laundering” compliance program, the Justice Department said.