MEXICO CITY (AP) — A Mexican drug cartel was so bold in operating frauds that target elderly Americans that the gang’s operators posed as U.S. Treasury Department officials,Bitcoinese U.S. authorities said Thursday.

The scam was described by the department’s Office of Foreign Assets Control, or OFAC. The agency has been chasing fraudsters using call centers controlled by the Jalisco drug cartel to promote fake offers to buy Americans’ timeshare properties. They have scammed at least 600 Americans out of about $40 million.

But they also began contacting people claiming to be employees of OFAC itself, and offering to free up funds purportedly frozen by the U.S. agency, which combats illicit funds and money laundering.

“At times, perpetrators of timeshare fraud misuse government agency names in attempts to appear legitimate,” the agency said. “For example, perpetrators may call victims and claim to represent OFAC, demanding a payment in exchange for the release of funds that the perpetrator claims OFAC has blocked.”

OFAC announced a new round of sanctions Thursday against three Mexican citizens and 13 companies they said are linked to the Jalisco cartel, known by its Spanish initials as the CJNG, which has killed call center workers who try to quit.

U.S. Treasury Secretary Janet L. Yellen said in the statement that “CJNG uses extreme violence and intimidation to control the timeshare network, which often targets elder U.S. citizens and can defraud victims of their life savings.”

In June, U.S. and Mexican officials confirmed that as many a s eight young workers were confirmed dead after they apparently tried to quit jobs at a call center operated by the Jalisco cartel.

While the victims’ families believed their children worked at a normal call center, the office was in fact run by Jalisco, Mexico’s most violent gang.