Four of these robberies have taken place in Sunnyvale, and no suspects have yet been arrested.

According to Det. Eric Fujii of the Sunnyvale Department of Public Safety, one to two men will approach a house, pretending to be hired yard workers for a neighbor. They then lure their victim into the backyard while accomplices rob the house.

The scams started on May 31 with two occurrences in Sunnyvale and one in Palo Alto. Since then, the suspects have made their way through Santa Clara and Milpitas and are now operating in Fremont. The latest incident in Sunnyvale took place on June 28.

Bill Veteran, a detective with the Fremont Police Department, said a half-dozen seniors in that city have been hit by the scam since early June. The victims have ranged from 70 to 90 years of age. One Fremont man had lived through World War I.

"This is a special breed of criminal," Veteran said. The scam artists have taken cash, jewelry, and in one Fremont incident, identification papers.

"The common thread is they have a line about needing to fix a tree or a fence in the backyard," Fujii said. However, the suspects alternate in numbers and use different vehicles, including a late-model blue Chevrolet Astro, a red van and a green sports utility vehicle. Veteran added that in the Fremont incidents, the suspects have pretended to be everything from water company workers to surveyors.

Victims fit a more consistent profile. Fujii estimated that the choice of elderly targets preys on their easy trust and confusion, though no one has been seriously injured. However, Fujii said one incident in Palo Alto involved a suspect throwing water on a female victim and telling her it was acid.

In one Fremont robbery, a suspect forcibly removed a portable phone from a female victim's hands and removed the batteries so she couldn't contact the police.

John Pilger, communications officer for the Sunnyvale Public Safety Department, says it's important to catch the scam artists. "There's always the chance of copycats," he says.

Because of this, residents should still look out for the suspects even if the crimes have ceased in the area, he said. Sunnyvale Police Chief Erwin Bakin mentioned several new characteristics of the crimes at the city council meeting July 15.

The robberies take place primarily on Saturday afternoons at single-family households and have involved asking for change for a large bill. The men then observe where the victim retrieves the money and signal their accomplices.

Pilger recommended asking for identification when any worker comes to the door. If they're with a utility, they're required to carry a photo ID. He said to keep in contact with neighbors about the potential presence of workers and ask them if there's any doubt about a worker's identity.

Because of the similarities between the victims, Fujii also mentioned that the suspects might have staked out their neighborhoods beforehand. He recommends caution when opening the door for anyone, especially if the person is drumming up business.

"Most places advertise in more conventional manners," he said. "If you suspect anything, just refuse to open the door and call 911."

Anyone with information can call Det. Fujii at the Sunnyvale Department of Public Safety at 408.730.7650.