When Linda and Ron Spence got a desperate phone call from someone who identified himself as their grandson, Michael, they didn’t think twice. He was in trouble. Of course they would help him.

Michael told the Spences he’d been gambling online. He owed money, his bank account had been frozen and he’d be thrown in jail if he couldn’t come up with cash to pay an attorney.

“Don’t tell mom and dad,” he begged the couple from Fort Myers, Fla. He said an attorney would call them to arrange payment, and hung up.

“I was worried about him. I couldn’t get hold of him,” said Linda, who’s 67. “I tried to get him on his cell phone, and no answer.”

The Spences ended up wiring $1,800 to the Dominican Republic, following instructions from another caller who said he was Michael’s lawyer.

Both callers were impostors.

The real Michael hadn’t been gambling, and he didn’t need a lawyer. He was at home in Phoenix, not in jail.

The Spences had been victims of the so-called “grandparent scam,” in which a con artist pretending to be a victim’s grandchild claims to be hospitalized or imprisoned and in need of emergency cash. The anxious grandparent rushes to wire money to the impostor or loads it onto a prepaid card and then hands over the PIN.

It’s an old con that’s seeing a resurgence nationwide. Instances of such impostor scams doubled from 2009 to 2013, costing Americans more than $73 million annually, according to the Federal Trade Commission.

The trend has drawn the attention of Congress, where lawmakers held a hearing about the problem Wednesday.

“It’s incredibly despicable,” said Sen. Bill Nelson, D-Fla., who led the hearing before the Senate Special Committee on Aging. “It preys on seniors’ willingness to do anything to help a family member in trouble.”

As in most such scams, there’s little hope of catching the perpetrators, much less getting the victim’s money back.

Scammers target seniors because they’re most likely to have “nest eggs,” own their homes and have excellent credit, said Joseph Campbell, deputy assistant director of the FBI’s Criminal Investigative Division.

Sen. Susan Collins, R-Maine, said the FBI and other law enforcement agencies needed to prosecute such scams more aggressively.

“Until we put in jail a lot of people who are committing these scams we are not going to see real progress,” Collins said.

Wednesday’s hearing produced no legislative solutions, but it was part of a push by Collins, Nelson and other members of Congress to bring attention to the problem and to pressure retailers and card makers to take proactive steps to protect customers.

“Private companies that sell prepaid debit products or offer wire services are the last line of defense for consumers before their money is sent, and then it’s lost forever,” Nelson said.

Looking back, Florida grandma Linda Spence can’t believe she fell for it. She retired as a civilian clerk from a sheriff’s office, and her husband, Ron, 73, used to work as a sheriff’s deputy in Denver. They’re far from naive about such scams.

But when the couple thought Michael was in trouble, their grandparent instincts kicked in, overriding their better judgment.

“You go into protective mode, like mother bear mode,” Linda said. “It’s ridiculous. I’m smarter than that, and I felt like a total fool.”