The police sketch of a slightly built young man wanted in a string of sexual assaults had circulated through Prince George's County neighborhoods for a few days when Cpl. Howard Calvert came face-to-face with a man who matched the description.

Calvert confronted the man, later identified as Canden Jovan Duncan, and chatted with him for a while last Thursday. When the man walked away, Calvert bent down and picked up a piece of evidence that police say has linked Duncan to one of the four crimes: a crumpled, discarded paper napkin he used to blow his nose.

Technicians with the county police DNA lab worked over the weekend to compare the samples on the napkin with evidence collected from the Feb. 25 rape of an Upper Marlboro woman. On Monday, based on what police say was a DNA match, a warrant was issued for Duncan, 21. He was arrested Monday and charged early yesterday with first-degree rape, first-degree assault, burglary and various handgun violations.

Detectives searched Duncan's Upper Marlboro house, which is in the neighborhood where three of the four attacks occurred.

Duncan was being held in the county jail last night without bond and is scheduled to appear in court for a bail hearing today.

Prince George's Police Chief Gerald M. Wilson praised the marriage of modern technology and old-fashioned police work. He said the Upper Marlboro community, frightened by the string of sexual assaults and robberies that began early in December, can "rest assured that a sexual predator of the highest order has been taken off the streets."

"We are obviously hoping to solve more than one case with this arrest," said Capt. Paul Evans, head of the department's Criminal Investigations Division.

In three of the four cases under investigation, a male attacker forced his way into the women's homes and, armed with a handgun, sexually assaulted them, according to police. Investigators said the suspect might have become familiar with his victims by posing as an alarm salesman. Before the Feb. 25 assault, the assailant asked to use the victim's telephone, saying he had walked his children to their school bus and had locked himself out of the house.

Evans credited the residents of the county's Marlton neighborhood for phoning in tips about Duncan, which led to the initial canvassing of his house in the 9200 block of Crockett Place. "It goes to show the importance of community response and how, when residents help, we can get the crimes solved faster," Evans said.