After two decades in the military, after earning two master’s degrees and navigating a successful career as a corporate coach, Victor Stemberger seemed ready for a peaceful retirement. But he had a new venture in the works.

Mr. Stemberger, of Virginia, had a $10 million inheritance waiting for him, according to men claiming to be affiliated with the Nigerian Ministry of Finance. Through a dizzying web of more than 160 emails over the course of a year, Mr. Stemberger, then 76, somehow grew convinced.

The final step to collect his millions was a good-will gesture: He needed to embark on a whirlwind tour to several countries, stopping first in São Paulo, Brazil, to pick up a small package of gifts for government officials.

With that parcel tucked away safely in his luggage, Mr. Stemberger got ready to board a flight to Spain, the next leg of his trip.

The next day, his son, Vic Stemberger, received a text from a Spanish number: “Your father is in prison.”

International criminals have long set their sights on older Americans, deceiving them with promises of money or romance and setting them up to unwittingly carry luggage filled with drugs or other contraband, hoping they will not raise flags in customs.

But Mr. Stemberger’s case shines a discomfiting light on a little-known program run by Immigration and Customs Enforcement known as Operation Cocoon, which is devised to disrupt international drug trafficking rings.

Under the program, ICE officials share information with foreign law enforcement agencies when they learn about potential smuggling. But critics say the program does not do enough to warn unwitting drug mules that they are being duped; instead, U.S. officials in some cases are delivering vulnerable older Americans straight into the hands of investigators in foreign countries, where they can be locked up for years.

“If somebody from the U.S. government showed up at my father’s house and spoke to my dad and said, ‘Hey, look, we have reason to believe you’re being scammed,’ there’s 100 percent no doubt he would have dropped it,” Vic Stemberger said.

His father has been in a Spanish prison since the police arrested him as he got off a plane in Madrid nearly two years ago and found more than five pounds of cocaine sewn into jackets in his luggage, according to court documents.

A Spanish court sentenced him last year to seven and a half years in prison.