At a 7-Eleven on Charles Street in Mount Vernon, a rack of Green Dot cards stands by the door, and manager Hirsh Choksi said they are popular. A MoneyPak card costs $4.95 and can be loaded with an amount ranging from $20 to $500. Choksi said $200 is a typical amount.

"It's easily accessible," he said. "Anybody can walk in; we don't have to check ID or anything."

For years, cigarettes and other goods were used as a basic unit of exchange in prisons and jails, said Stephen E. Lankenau, an associate professor at Drexel University's School of Public Health who in 2001 studied the cigarette black market at 16 detention facilities.

"I saw them using things such as potato chips or bars of soap, things available at the commissary," he said. "Now it sounds like it's moving to more sophisticated alternatives. It doesn't really surprise me. You can come up with some clever ideas when you're sitting inside a prison cell."

Without electronic transfers, moving cash around was a dirty business involving rubber gloves, lubricant and body cavities, according to retired Florida warden Ron McAndrew.

If an inmate wanted to transfer money to the outside, he would have to pass it to a visitor to smuggle out, McAndrew said. Cash was also vulnerable to detection — McAndrew said he once seized almost $6,000 from an inmate, money that represented the proceeds of almost a decade inside.

Scattered news accounts point to the growing use of such cards — both as a method of exchange and as a way for criminals to extort money from their fellow inmates' families.

This year, for example, the Atlanta Journal-Constitution reported that a woman was sent text message pictures of her brother, bloody and beaten, followed by a message from someone in a prison in Michigan. The sender said he would kill the woman's brother if she did not send $1,000.

The request: Put the money on Green Dot cards. Authorities there said the problem circles back to contraband cellphones, without which illegal activities would be harder to coordinate.

In a 2008 federal indictment that established the BGF's far-reaching influence over Maryland correctional facilities, federal agents wrote that the gang offered "protection" to newly arrested detainees by requiring them to pay money through prepaid credit cards. Those who did not pay would be targeted for violence.

Such extortion plots already existed through more traditional transactions. Maryland prosecutors in 2008 brought charges against a former corrections officer named Fonda Deneen White and her boyfriend, an inmate and high-ranking Black Guerrilla Family member Jeffrey L. Fowlkes.

The two pleaded guilty to depositing 180 checks, money orders, cashier's checks and postal orders into White's personal bank accounts — all from prisoner bank accounts and family members of prisoners.

The case began when an inmate's mother told the FBI she received threatening phone calls from inside the prison in 2007, demanding money in exchange for her son's protection.

"You're not gonna turn this into a game, cause it's a game you don't want to play," White said on one call. On another: "We understand violence."

When Drug Enforcement Administration agents in July 2010 raided the home of Alicia Simmons, a corrections officer later convicted of being a contraband smuggler for the BGF, among the items in her home were Green Dot cards in the names of both Fowlkes and White.

One woman who recently contacted The Baltimore Sun described sending money to her son using Green Dot cards while he was in the Baltimore jail facing an assault charge two years ago. The son said he needed the money to buy cigarettes. She said she sent $50 to him three or four times by calling a cellphone number and giving the man who answered the number.

"It was somebody inside that jail," she said.

Eventually, the woman, who asked not to be named because she fears retaliation, stopped sending money. "He said, 'Mama I need that money, I have to have that money.'"

One employee who works at the Baltimore Detention Center, who spoke on condition of anonymity for fear of being fired, said such cards are ubiquitous behind bars. They are used for every transaction that occurs, whether between two inmates or between an inmate and a corrections officer, the employee said.

"If they proposition me to bring something in, they'll say, 'I'll give you $150 to bring this cellphone in,'" the employee said.