Pennie Duff, 23, has spent the past few nights at a friends house, monitoring a police radio in a room darkened by drawn blinds and masked windows.

Theodore Rice, a retired car salesman who has lived 57 years here in rural Calvert County finally went out and bought a lock to secure the screen door to his home.

"This whole thing got everybody stirred up," said Rice. "I've heard of things going on in the country, but I can't remember anything this bad happening."

This "thing," as Rice put it, ended yesterday morning with the capture of the last of three suspects wanted in connection with Friday's holdup and slaying at the Knotty Pine Bar and Grill here. A bartender was shot and killed during the robbery.The owner of the bar was shoot in the mouth and his wife was pistol whipped . One suspect kidnaped a woman during his escape and held her, her husband and two small children hostage for several hours before fleeing during the night.

For Calvert County residents like Pennie Duff, it was a stage of siege while the men were at large.

It was also a reminder - of which there have been more and more in recent years - that tranquil Calvert County, known for its quiet woods, tobacco farms and beach towns, is on the brink of becoming "citified," in the words of Nancy Bowen, 30, a lifelong county resident.

"I see reports of this sort of crime on TV all the time," said Viola Foote, 55, whose husband is recovering from the bullet wound in the mouth at Calvert Memorial Hospital. "But it's just something we don't see much of here at home."

There is no crime wave here, although crime is increasing along with the population, which has one of Maryland's highest annual growth rates, 5 per cent. The population as of 1975 was about 25,000.

One of the suspects charged was from Landover and two gave District addresses. To some extent, this fact is reassuring to some county residents.

"These were definitely outsiders," said Susan Axley, 28, who was held hostage by one of the suspects. "The people in Calvert County are good people."

In fact, said State Police Sgt. Roland Haymann, who has investigated crimes in the county for 12 years, the bulk of the larcenies, burglaries and other less violent crimes are commited by people who live in Calvert County.

Armed robbery, he said, is different. He estimated that half of the 30 armed robberies he has investigated in recent years involved outsiders - most from the District and Prince George's County.

"It doesn't take that long to get here from, say, Washington," said State Police Lt. Joseph Mihm. "Criminals know that there will be less police coverage in a small rural county like Calvert."

There are only two main routes that run in and out of the county. Combined routes 2 and 4 extend directly from the District to Solomons, the southern-most town in Calvert. Rte. 231 crosses the Patuxent into southern Prince George's County. The direct route from Washington did not exist until 1967.

Within two years, the new Lower Patuxent Bridge will connect the southern tip of Calvert with St. Mary's County and dissolve the isolation that section of Calvert County has enjoyed since settlers arrived in the 17th century.

Police say they are concerned that the bridge will not only provide another route for criminals, but will attract a large flow of people from the Patuxent Naval Air Test Center in St. Mary's County.

"That means we're going to start getting porno shops and fly-by-night auto dealers and all kinds of problems that come from a military base," Hayman said.

Among the county's older residents, who see the once-enormous sharecropping farms turning into subdivisions for housing that will eventually attract more newcomers to Calvert, there is nostalgia for the more secure past days when neighbors knew each other and settled differences among themselves.

"I guess we could put up roadblocks so certain people couldn't get into the county, but that just wouldn't be realistic," said lifelong Calvert resident Marry Harrison.

Police arrested two suspects shortly after the shooting but the third - who briefly held the family hostage - remained at large until early yesterday morning.

He was captured by state police about 7:30 a.m. off old Bayside Road below Chesapeake Beach after he was spotted on the porch of a nearby house after an all night manhunt. The suspect was shot in the leg by police before he surrendered.

Charged with murder and armed robbery in connection with the incident were Walter West, 24, of Landover, and Tony Mitchell, 26, and Farrakhan Bey, 28, both of the District.