On a leafy, quiet street in Silver Spring, an air conditioner hangs from the roof of a house by a rope.

The house and the air conditioner belong to Marlene Arcos, a single mother and house cleaner. The rope belongs to the contractor she hired to construct an addition to her home. He left her and the air conditioner hanging, she said, bilking her of $75,000 for work he never did.

"He left me with a mess," Arcos said. "I have no place to go."

Instead of putting on an addition, the contractor caused structural, electrical and foundation problems before abandoning the property and leaving the house exposed to the outdoors. Rodents come and go as they please. Last summer it was 110 degrees inside, but Arcos and her daughter stayed. They had no choice. She is trying to figure out how they will handle the approaching winter.

Arcos turned for help to Montgomery County's consumer affairs division, which has nine investigators who look into more than 2,000 cases of fraud every year, from illegal home contracting to scams for so-called green card lotteries. One woman once passed herself off as a travel agent and sold bogus e-tickets for holiday air travel to hundreds of immigrants.

"Everyone who works here is riled up by some sort of injustice," consumer affairs chief Eric Friedman said. "To have a job where you can see the world divided into right and wrong, you can feel like you are always taking the right side of an issue."

A sign on his desk says: "The Bogus Buck Stops Here."

The department brought in its bulldog for the Arcos case: John B. Creel Jr., a fast-talking, fast-walking investigator with 24 years of experience and thousands of cases under his belt. He describes himself as "unfortunately type-A." He refers to his targets as "scoundrels."

"We often wonder what kind of performance-enhancing substance he puts in his orange juice in the morning," Friedman said.

Creel employed a strategy in the Arcos case that has become increasingly popular in the county: He filed criminal charges against the contractor for working without a license, for abandoning the property and for felony theft. The contractor recently pleaded guilty to the charges, he said, and is supposed to pay Arcos back.

"That was a case that really stopped me in my tracks," Creel said. "It's shocking what this woman is going through."

In the past two years, consumer affairs actions have led to the conviction of 10 people for similar crimes. Warrants are open on 11 more. Three others are awaiting trial.

"People can't afford these scams and be left with a mess on their hands," said County Executive Douglas M. Duncan (D). "They have no way out of it. Some of these stories just break your heart."

While the investigators see their share of used-car scams, bogus charities and businesses that open for a few days before vanishing, recently they have focused on home improvement contractors.

With the cost of real estate skyrocketing in the county, many people are fixing up their homes, either because they can't afford to move or because they are getting ready to and want to increase their home's value. Enter the fake contractors, with their assurances that they are licensed, respected and fast.

Creel recently had one man prosecuted for bilking seven people. After taking thousands of dollars to do landscaping work, the man cleared some dirt and never came back.

In another case, a woman signed a contract to do $10,000 worth of work on a kitchen. She cashed the check and never came back.

And then there's the man who promised to work on chimneys. He would take the checks and then come back a couple of days later to say they had been lost in the laundry. Or that they blew out of a window. The victims would write more checks, and then he would cash both, never returning to do the work.

Creel and other investigators said they have witnessed a disturbing trend recently: Hispanic scam artists preying on other Hispanics. That was what happened in Arcos's case. The contractor spoke Arcos's language, and she didn't think to ask for proof of licenses.

"I trusted what he said to me," said Arcos, who sometimes struggles with English. "I could understand him better, and he could understand me. I don't know why he did this to me."

Friedman said he is adding a bilingual investigator to his team.

Investigators are also stepping up their protection of the elderly, who are frequent targets because con artists think they will be scared to come after them. But not when Creel is on the case. Creel takes a county car to the victims' homes, helps them into their wheelchairs, if necessary, and drives them to court.

The con artist is generally not happy when the elderly victim shows up in court.

"That's a weapon of mass destruction in front of a judge," Creel said. "The judges go berserk when they see the victim there."

The investigators acknowledge that as long as there are people there will be scams. The scams evolve, and they frequently are linked with things happening in the news or culture.

Creel thinks it won't be long until he's investigating scams promising lower heating bills, particularly in light of rising energy prices. The tightening of the nation's borders has prompted dozens of scams; investigators recently took down a sign that promised passports in 24 hours.

"You can't get a passport in 24 hours," Friedman said. "It's impossible."

Said Creel: "Unless you're the secretary of state."

Marlene Arcos uses a temporary sink in her kitchen. The contractor bilked her of $75,000 for work on her house that was not finished, and he is supposed to pay her back. Investigators have been focusing on home improvement fraud.Creel examines the furnace in Arcos's home. He said it is too small for the ducts. Arcos is trying to figure out how she will handle the winter.County investigator John B. Creel Jr. is handling Marlene Arcos's case. The contractor pleaded guilty, Creel said.Daylight shows through an outlet hole inside the addition at Marlene Arcos's house in Silver Spring. At left is the building permit posted more than a year ago. "He left me with a mess," Arcos said of the contractor. "I have no place to go."