A robber who carried a sawed-off shotgun to demand cash and a plastic bag to collect it in apparently has held up four stores in Montgomery County in the past two months, county police said.

The most recent robbery was last week; the first was Oct. 12. In one of the four, a second robber may have remained outside the store.

The robberies occurred between 11:30 p.m. and 4 a.m. In all four, county police said, the robber showed a sawed-off shotgun and demanded money from the store clerk.

In at least three cases, police said, the robber brought a white plastic shopping bag in which the money was to be placed. The robberies involved two convenience stores and two drugstores.

Meanwhile, three convenience store robberies were carried out in Northeast Washington in the early hours of Nov. 27 and Nov. 28. One involved three robbers, the others two. One of the convenience stores, a 7-Eleven in the 2500 block of Benning Road NE, was held up on both days.

In all three cases, one robber jumped over the counter and demanded the money from the cash drawer. The robbers were armed in each of the Northeast cases, according to police.