The con starts as a pop-up message or a letter that carries a dire warning from the federal government. It ends with unwitting victims delivering bundles of gold to international criminals.

From Illinois to Oregon, thieves are bilking millions from elderly Americans by posing as government officials and convincing victims that their life savings are at risk. The only way to avoid more trouble is for the victims to liquidate their cash for gold and hand it over for safekeeping.

That’s when contact with the supposed official goes dark — and the victims are left scrambling for answers.

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“The harsh reality is, the vast majority of cases where people are a victim of a crime like this, they never get their money back,” said John Breyault, who oversees fraud trends for trade group National Consumers League.

Mr. Breyault said organized crime groups in Jamaica, West Africa and Southeast Asia are primarily behind the sophisticated schemes to defraud Americans.

Gold is an ideal haul for bandits because it’s nearly impossible to track once it has exchanged hands with the criminal associates, Mr. Breyault said.

Federal data show thieves are cashing in on the con. The FBI reported that victims lost more than $55 million in various precious metals schemes between May and December of last year.

Police departments from Brentwood, California, to St. Louis, Missouri, have warned residents about the fraudsters and how to avoid forking over their savings.

But even with law enforcement’s best efforts, horror stories are found throughout the country.

• An Oregon couple in their 70s paid a con man nearly $280,000 in gold in June. Federal prosecutors charged Navjot Singh, 34, in the incident.

• An elderly man from Martin County, Florida, paid a scammer more than $260,000 in gold bars in May. Tejaskumar Patel, 21, of Chicago, was charged with the theft last month.

• A New Jersey woman in her 70s gave swindlers more than $100,000 in gold in February. Police charged Anil Mangukia and Yash Navdiya, who are Indian nationals and U.S. green card holders, in connection to the theft.

• A Colorado woman in her 70s handed over $120,000 worth of gold to scammers last year. Police charged Sagar Patel, 26, of New Jersey, in February after tying him to the theft.

• Prosecutors in Montgomery County, Maryland, said they charged Vipul Thakkar, 52, last week with swindling more than $1 million from a 74-year-old Bethesda man with memory loss issues.

Prosecutors said Mr. Thakkar was on his way to obtain another $240,000 in gold from the man when the victim saw a story on CBS affiliate WUSA about a woman losing $1 million of her own money to a precious metals scam. The victim called police out of concern that he, too, was being defrauded, according to the station.

Police set up a sting to catch “Mike Rodgers,” who prosecutors allege was Mr. Thakkar. The suspect was arrested right after the next gold drop-off.

Mr. Thakkar was charged with multiple theft offenses related to the scheme. He’s being kept behind bars until his September court date.

Court documents said Mr. Thakkar admitted to receiving a 2% cut for acting as the courier. It wasn’t clear who he was transporting the money on behalf of, but prosecutors said he was working with people in India to carry out the fraud.

Maryland’s most populous jurisdiction has become a gold mine for scammers. Montgomery County State’s Attorney John McCarthy said there have been nearly two dozen reports of similar schemes so far this year.

That includes 19-year-old Zhenyong Weng, from New York, who was accused last month of tricking a Silver Spring woman into giving him $900,000 in gold after he claimed Russia was trying to take her money.

“People really need to be on guard. If you have an elder loved one, talk to them about this, spread the word, that’s how we’re going to combat this problem,” FBI supervisory special agent Keith Custer told Fox affiliate WTTG in March. “You really need to calm down, listen to what’s being told to you, take a breath and don’t act. Hang up the phone, talk to someone you trust.”

Some of the frauds are so elaborate they can be hard to sniff out.

Ohio retiree Lori English told AARP she was fooled by a trio of scammers who originally came up as “Social Security Administration” on her caller ID. The scammers, posing as government officials, said her identity had been stolen and cartels were using it to launder money and traffic drugs.

Ms. English, 54, was informed she could be indicted in a criminal case — so she needed to empty out her savings account, trade it in for gold and hand it over or risk being frozen out.

The victim said she first transferred half her life savings, which amounted to $250,000. Months later, the thieves pushed her to forfeit the rest.

The scammers told her that she was in the clear afterward and that she just needed to fill out some paperwork at her local police station. But her story left authorities confused. That’s when she said she started fearing the worst.

“I’m driving home, my phone rings, and it’s a strange phone number, and there’s a strange male voice that says, ‘You’ve been scammed.’ So I scream, I hang up the phone, and I call 911,” Ms. English said.

But she caught a massive break: Two of the couriers were nabbed in Oklahoma a week after she notified police. Officers found $240,000 in gold with her receipt attached when the suspects were arrested.

The good fortune may have persuaded Ms. English to share her experience.

Mr. Breyault of the National Consumers League said many victims don’t report fraud because they’re ashamed of being conned. He said the FBI’s figure of $55 million being stolen is likely a conservative estimate in that light.

The fraud expert said no one from the local or federal government will ever phone you to demand you need to purchase gold. If you ever do get such a call, he said, it’s best to assume someone is trying to steal from you.