Montgomery County police continued their search yesterday for a third suspect in the July 4 slaying of a 20-year-old college student, while authorities in Prince George's County and the District investigated possible links to similar "mailman" robberies in their areas.

A reward of up to $1,000 is being offered by Montgomery's Crime Solvers program for information leading to the arrest of Earl W. Davis, 20, of the 200 block of V Street NE, Montgomery police spokesman George Ludington said yesterday.

Davis, who police allege shot and killed Anthony F. Williams during a struggle in Williams's Silver Spring town house, had been placed on one year's probation May 22 after pleading guilty to drug possession charges in Montgomery, court records show.

Williams, a student at Montgomery College, was fatally wounded in the abdomen about 10:15 a.m. after he answered his door for a man posing as a postal carrier. Police allege that Davis and two other men rushed into Williams's home and demanded money.

Williams's cousin witnessed the shooting, but was unharmed by the suspects, who ran away, police said.

Two 19-year-old men are being held without bond in the county jail on murder charges.

Adrian M. Brockenberry, of Northwest Washington, was arrested last week after his fingerprints were allegedly found on a bogus mail package used in a similar Prince George's robbery.

Police allege that Brockenberry was dressed in a postal carrier's uniform during the July 4 homicide.

Also in custody is Kodie C. Kinzer, a former schoolmate and neighbor of Williams's, who surrendered to police Thursday.

Police said Kinzer and the victim attended Walt Whitman High School in Bethesda and played basketball together. Kinzer, of Wheaton, had lived one block away from the victim, police said.

Meanwhile, District detectives are investigating an "obviously similar" May 30 robbery on Girard Street NW, a D.C. police spokeswoman said.

In that case, a 28-year-old man was left paralyzed after he was shot during a struggle with robbers, one of whom posed as a postal carrier. The suspects fled with a videocassette, camera and $120 in cash, police said.

The Prince George's "mailman" robberies occurred June 25 and July 2. There were no injuries in those robberies, police said.