Montgomery County police, reporting that the county has been hit hard this summer by nomadic families that generally prey on the elderly through robberies and home improvement scams, are warning homeowners to be on the lookout for such groups.

These nomadic bands hit the Washington area every year, law enforcement officials said. They are loosely organized groups of men, women and children who offer to make minor, inexpensive repairs on homes, but later tell the elderly homeowners that costs have run higher. The also frequently rob the homes.

"It's a periodic problem throughout the metropolitan area as far north as Frederick," said Sgt. Gary Gardner, spokesman for the Howard County police department.

But this summer, the problem has been particulary acute in Montgomery, where 16 such thefts have been reported since mid-May, police said. Seven of the cases occurred in Bethesda, four in Silver Spring and five in Wheaton, reports show.

Most of the recent Montgomery County cases, as well as a "handful" of similar incidents in past years in neighboring Howard County, have involved home improvement schemes, police said. The number of reported thefts is believed to be low, police said, because frequently the crime victims are embarrassed or frightened and therefore do no tell police of the episode. They also sometimes do not realize anything has been stolen until it is so late that a connection between the home repairs and the theft is not drawn.

In many of the incidents, the victims have been older residents, ranging in age from 77 to 92 years old, police said. "While one suspect is asking for hot water for the cement mixer, another . . . is in the house stealing money and jewelry," said Lt. Fred Ailes, a detective in the Bethesda district station.

Victims in Montgomery have reported jewelry and other valuables stolen worth as much as $10,000, authorities said.

Trying to apprehend the nomadic bands is very tough, according to police. "We have to get lucky to catch them in the act," said investigator William Patterson of the Montgomery County police Repeat Offenders unit. "Once there is an inclination that they may be known, they're gone."

The thieves have a strongly defined crime pattern, Patterson said. Each spring, bands of the home improvement workers travel north from Florida, Alabama and South Carolina, he said. The groups, usually from the same family, can number as many as 17 members and use more than 130 aliases, he added.

Patterson said the bands stay in the Washington area for one to two weeks before moving on to New York, Illinois, New Jersey and Michigan. In the late fall, they return south, leaving behind another crime trail, he said.

The bands are attracted to the Maryland suburbs because of the promixity to major highways, such as I-95 and I-270, and the abundance of social service agencies that can provide them with welfare assistance such as food stamps and medical care, Patterson said. Many of them live in motels along Route 1 in Prince George's and Howard counties, he said.

The home improvement scams are relatively simple, police said. The nomads canvass neighborhoods in cars or knock on the front doors of unsuspecting victims, usually targeting the elderly, Ailes said, and approach a homeowner with an offer for inexpensive home improvement jobs, such as fixing a leaky roof or paving a driveway.

While completing the repairs with shoddy materials, the workers also are looking for opportunities to steal cash and jewelry from houses, Patterson said.

Montgomery County police believe several groups of the nomads are responsible for the recent rash of thefts, according to Patterson. Bethesda investigators have eight different suspects in its seven cases, Ailes said, but none has been apprehended.

Patterson said police seldom recover property stolen by such thieves, and victims often don't discover the theft until weeks after it has occurred.

To protect themselves against such scams, police advise homeowners to avoid home improvement "deals" from unlicensed or unbonded contractors. Police also advise them to check with the Better Business Bureau before authorizing work and to report immediately suspicious activity in their neighborhood.