The surveillance program, overseen by investigators with U.S. Immigration and Customs Enforcement, collected records of any money transfer greater than $500 to or from Mexico, Sen. Ron Wyden (D., Ore.) said in a letter sent to the DHS inspector general. It also collected information on domestic or international transfers exceeding $500 to or from the states of Arizona, California, New Mexico and Texas.

The briefing was the first time Congress was made aware of the program’s existence, according to Mr. Wyden’s letter.

t couldn’t be determined exactly how authorities were using the data at issue, which appeared to solely involve money-transfer services rather than banks. Such services are popular among people who don’t have bank accounts and are commonly used to send money abroad, such as to family members back in an immigrant’s country of origin.

The surveillance program’s origins date to at least 2010, when the Arizona attorney general’s office began collecting similar data from Western Union following a money-laundering settlement, according to Mr. Wyden’s letter.

Under the program, ICE investigators collected approximately six million records of international money transfers from Western Union and Maxitransfers since 2019, the letter states.

Dozens of other money-transfer companies have voluntarily shared data through the program, the letter said.

“I cannot think of another arrangement like this that combines an illegal use of subpoenas with an inscrutable, private nonprofit just for the purposes of housing this information,” said Nathan Freed Wessler, senior staff attorney with the American Civil Liberties Union.