The Consumer Financial Protection Bureau on Friday sued the operator of the Zelle payment network and the three U.S. banks that dominate transactions there, saying the firms failed to properly investigate fraud complaints or provide compensation to victims.

The CFPB said customers of the three banks – JPMorgan Chase, Bank of America And Wells Fargo – have lost more than $870 million since Zelle launched in 2017.

Zelle, a peer-to-peer payment network from the bank-owned fintech company Early Warning Services, enables instant payments to other consumers and businesses and has quickly become the largest service of its kind in the country.

“The country’s largest banks felt threatened by competing payment apps and rushed to exclude Zelle,” CFPB Director Rohit Chopra said in a statement. “By failing to provide adequate security, Zelle became a goldmine for fraudsters, while victims were often left to fend for themselves.”

The lawsuit is the latest move by the CFPB in the final days of the Biden administration. Many of the measures taken, including steps to cap late fees and overdraft fees on credit cards, were met with strong opposition from banks and their trade groups. Companies have successfully defied regulators by choosing venues deemed appropriate for lawsuits challenging federal oversight.

In fact, JPMorgan said in August that it was considering litigation against the CFPB if the regulator tried to punish the bank for its role in the Zelle network.

Zelle said in a statement Friday that it is prepared to defend itself against this “meritless lawsuit.”

“Zelle is a leader in the fight against fraud and fraud and has industry-leading reimbursement policies that go beyond the law,” said Jane Khodas, a spokesperson for Zelle. “The CFPB’s misguided attacks will embolden criminals, cost consumers more fees, stifle small businesses, and make it harder for thousands of community banks and credit unions to compete.”

This story is developing. Please check back for updates.