BRENTWOOD -- An elderly Brentwood couple was victimized Wednesday in what's become known as a "grandparent scam," police said.

The Summerset residents received a call from a man who identified himself as a police officer and said that their granddaughter had been arrested when cocaine was found in her car during a traffic stop.

The caller told them the young woman wanted to post bail but didn't have the money.

He instructed the couple to go to a CVS drugstore and buy a Green Dot MoneyPak, which can be used like a gift card.

The victims then gave the thief the security code on the back of the card, thereby making the funds available to him.

Fraudulent use of MoneyPaks has been on the rise in recent years, and it's virtually impossible to track down the con artists, said Brentwood police Detective Walter O'Grodnick.

"Once that money's gone, it's gone," he said.

And MoneyPak is just one avenue that thieves use to defraud the elderly after gaining their trust: O'Grodnick says that pretending to be a family member to gain access to a victim's assets is common enough that the crime is known as the "grandparent scam."

The caller will ask whoever picks up the phone if he or she knows who is speaking, which often prompts the unsuspecting target to divulge the name of a relative with a similar-sounding voice.

Armed with that nugget of personal information, the perpetrator will pose as that person and ask for money to get out of some predicament.

"We deal with these type of incidents all the time," O'Grodnick said.

His advice to anyone who uses MoneyPak cards is simple: Don't share the security code with anyone.

To protect oneself from fraud in general, don't offer any personal information over the phone, O'Grodnick says -- not your driver's license number, Social Security number, debit or credit card numbers, date of birth or full name.

And if you start to have misgivings while talking to a stranger, hang up and tell someone you trust or alert the police, he said.

"That's the biggest thing we try to stress," O'Grodnick said.

Contact Rowena Coetsee at 925-779-7141. Follow her at Twitter.com/RowenaCoetsee.