Another spate of counterfeit money has Suffolk police tracking down leads and working with the Secret Service to find the source of the bogus bills.

Three fake bills were passed on Friday and another on Monday, according to Suffolk spokeswoman Debbie George.

The new rash of phony money comes during a period of increased activity by counterfeiters. Late last month, police said that reports of counterfeits had almost doubled compared to the same period last year.

At about 9 p.m. Friday, police were called to the Pure Station on Holland Road when a clerk discovered that a $100 bill that had been received there earlier was a counterfeit.

“This one is somewhat interesting,” George said of the incident, noting that while a police officer was on the scene for his investigation, another customer entered the store and attempted to pay with a $50 bill that was determined to be counterfeit.

Officers took the customer to police headquarters and questioned him, but he was later released.

“Police have to be able to prove that the individual knew the bill was counterfeit when it was passed” in order to press charges, George said Monday.

She added that there is reason to believe the two Pure Station counterfeits were related. During his interview with police, the customer described the person who gave him the counterfeit $50 bill. His description was similar to that of the person who had given the clerk the $100 bill, George said.

Within an hour of those incidents, a bogus $20 bill was reported at the Neighborhood Supermarket. Police also interviewed and released the person who passed that bill, she said.

And on Monday, she said, someone attempted to use a fake $100 bill to pay a bill at the Suffolk Department of Public Utilities.

That person was detained and released pending an investigation. He told police he had received the money in a rent payment from a tenant.

Counterfeit bills have turned up in Suffolk in denominations ranging from $100 all the way down to $1, police say.