In 11 weeks there have been11 home invasions and one burglary at locations scattered across the entire region.
The victims were all Asians who own or operate businesses within the a relatively small area of West Philadelphia and Upper Darby.
"We believe the offenders are clearly looking at business owners, they target and wait for them to close," said Lt. John Walker of the Philadelphia Police.
Why Asian business owners? Authorities say criminals believe Asian merchants are more likely to carry large sums of cash and have cash in their homes.
Assistant Delaware County D.A. Pearl Kim adds the victims are less likely to notify police and prosecutors.
The head of the local Asian Federation, Mahn Suh Park, says with some new imigrants that perception could be reality. Newcomers are more comfortable with cash.
It's cultural, he says.
"They don't trust authorities - the banks, IRS, immigration, and up to the police," Park said.
On Thursday, Delaware County D.A. Mike Green said he is trying to reach out to Asian businesses, stressing that keeping cash can make you a target.
"It absolutely makes them a bright shining light for those who want to victimize folks who have significant amounts of cash," Green said.
Feds are joining the case. On Wednesday, the FBI announced a $10,000 reward.
"Somebody knows something and we're hoping that $10,000 shakes some information loose for us," said Special Agent J.J. Klaver of the FBI.
Authorities want this to stop soon. They have only sketchy descriptions of the masked assailants who, so far, have made few mistakes and left little in the way of clues.