Police call it a "Flim Flam" robbery.

In this unusual but common crime, a female undergraduate voluntarily gave up $200 and jewelry as collateral for a fictional reward that two other woman had promised she would receive.

The Harvard University Police Department (HUPD) said two female suspects approached the student on Mill Street last Thursday and told her they had found a purse that allegedly contained cash.

The suspects told the student that they all would receive a reward for finding the purse if they could produce $1000 in cash.

The victim led the suspects to a bank in Central Square where she withdrew money from her savings account, said Detective Sergeant Richard Mederos, HUPD's lead investigator.

The suspects told the victim to enter the bank and collect the award. She did, and the suspects fled, "never to be heard from again. It's a classic," Mederos said.

An HUPD community advisory describes the suspects as black females both standing 5 feet 6 inches tall. One of the women called herself "Linda" and has very short black hair and a thin build.

The other suspect called herself "Janet" and had a "bob style" haircut, the advisory said.

Mederos said that although infrequent, Harvard students have been victimized in a similar manner before.

HUPD spokesperson Peggy A. McNamara outlined the paradigm that Flim Flam robberies usually follow.

"It's what's called a 'pigeon drop.' One person will approach you and say they have a wallet...that has a huge amount of money in it and try to persuade you to go to a bank...or a credit union. They go into the bank...and come out and say you have to give them money [to get the reward]," McNamara said.

The crimes are usually solvable only if the victim is able to identify the suspects from a line-up, but prosecutors find Flim Flam robberies difficult, said Frank Pasquarello, the Cambridge Police Department's spokesperson.

However, Mederos said that since victims usually cannot remember faces well, they have no way of identifying the suspects. The victim in this case could not describe the suspects well enough for composite sketches to be drawn.

Both Pasquarello and Mederos offer students a simple piece of advice: do not always believe what people tell you.

"If you think of it logically, if some one finds 500 dollars on the ground, why would they give it to you?" Pasquarello said.

Mederos said the victims "fall for it because the individuals perpetrating the fraud are credible."

"They're catching them at a vulnerable moment in the community...with their minds focused on their studies," Mederos said.

"Very few [people] come up to people on the street and offer them money," he added.