The Fourth of July slaying of a 20-year-old Silver Spring man by a group of robbers, one of whom posed as a postal carrier, may be linked to at least two recent robberies in Prince George's County, police said yesterday.

In each of the three incidents, a man wearing a U.S. Postal Service uniform, joined by one or two others, and armed with an Uzi semiautomatic pistol knocked on doors in the middle of the day, and once inside, demanded money and jewelry.

On Wednesday, Anthony F. Williams, a student at Montgomery College, was fatally shot after he answered a knock on the door of his town house in the 2400 block of Michigan Avenue. When Williams opened the door, three men, one dressed in a postal uniform, forced their way into the house.

Williams was shot once in the abdomen during a struggle with the men, police said. Williams's cousin, asleep on a couch, was confronted by the gunmen who demanded money, authorities said. Told there was no money, the robbers searched the house and then ran away. Williams's cousin was unharmed, police said.

The latest incident in Prince George's occurred Monday, when three armed men, one dressed in a mail carrier's uniform, robbed a house at 2:23 p.m. in the 10400 block of Parapet Court in Fort Washington, said Cpl. Chuck Cooke, a police spokesman. In that case, a man was ordered to lie on the floor while the robbers took jewelry and money from the house, Cooke said.

On June 25, two robbers used the same ruse to rob and terrorize two men in the 4200 block of 28th Avenue in Temple Hills, Cooke said. One robber wearing a postal carrier uniform carried a silver handgun, while his accomplice was armed with an Uzi semi-automatic gun, police said.

During that robbery, which occurred at 11:30 a.m., the assailants tied up their two victims in a bedroom and demanded money and jewelry. After ransacking the house, the robbers fled in a stolen car, which they abandoned about a block from the victims' home, according to Prince George's County police records.

Montgomery County police said robbery apparently was the motive in Wednesday's shooting. Police have not determined what may have been stolen from the house.

No shots were fired in the Prince George's County cases, police said. Authorities speculate that Williams may have been killed because he resisted. "It does not appear that the victim was the target," said Sgt. Harry Geehreng, a Montgomery County police spokesman.

Police in Montgomery and Prince George's have developed similar descriptions of the gunmen in the three cases. Authorities have identified the suspects as young men ranging in age from 16 to 22 years old.

Impersonation of mail carriers is a rare occurrence, although there have been several instances where letter carriers have been abducted and their uniforms and vehicles stolen, said Postal Inspector Tom McClure.

Postal uniforms can be readily obtained from private uniform stores, pawnshops and former letter carriers, postal officials said.

Although Wednesday was a federal holiday and regular mail was not processed, letter carriers deliver express mail, special delivery packages and perishable items 365 days a year, postal officials said.

McClure said homeowners should demand identification under suspicious circumstances.