Concerned by brutal armed robberies at three Montgomery County homes, police said yesterday they are investigating whether the crimes are part of a pattern that includes other surburban counties and residential areas of upper Northwest Washington.

Sgt. Harry Geehreng said Montgomery police are studying at least three other robberies this year in the county to see if they are related to the two in Silver Spring Tuesday night and another in Chevy Chase the next night. In all six robberies, two armed men in their twenties, of medium build, broke into the homes of older people through a rear window or door, according to Geehreng.

"The hour of the night -- late evening -- is the same. All of the victims so far have been up in age in their seventies," Geehreng said, noting that the one exception was the 40-year-old wife of a 77-year-old man robbed Wednesday night.

The six single-family residences were ransacked and most of the residents were tied up, made to lie on the floor, or locked in rooms, he said. The robbers took money or jewelry.

District police officials were quick yesterday to rule out similar robberies in the city.

"As far as our investigations are concerned, we have not determined at this point that any armed burglaries committed in the District of Columbia are associated with those in Montgomery County," said Lt. William White III, a Metropolitan police spokesman.

Geehreng said Montgomery police are seeking a meeting on the robberies with investigators from the District and Fairfax and Prince George's counties.

In the Silver Spring robberies, a 77-year-old woman was beaten when two men broke into her home on Devon Court and a couple living across the street was locked in the bathroom while two men looted their home. The following night, two men broke through the door of a home in the 4700 block of Chevy Chase Boulevard and tied up a woman while they ransacked the house. Her husband was not disturbed while he slept.

Among the other Montgomery armed robberies being investigated, according to Geehreng, are:

Jan. 21, in the 10700 block of Huntwood Drive, Silver Spring, two men broke into a house shared by two elderly women, knocking one woman to the floor and covering the face of the other with a pillow.

Feb. 22, in the 2600 block of Blaine Drive, Silver Spring, two men kicked in the kitchen door of a house, forcing four residents to lie on the floor.

April 17, in the 1300 block of Woodside Parkway, Silver Spring, two men broke a window and forced open a kitchen door while an elderly couple watched television. The robbers forced the couple to lie on the floor.