Using DNA evidence found on a pair of women’s underwear more than 30 years old, the Manhattan district attorney’s office identified a man suspected of more than 21 violent sexual assaults and burglaries in the suburbs surrounding Washington, D.C., and other areas, the D.A., Robert Morgenthau, said yesterday.

The alleged “Silver Spring Rapist,” Clarence Williams, 58, had been charged in the 1970s with two violent rapes in New York City. After a hung jury in his trial for the 1973 rape of a 25-year-old artist and actress at Chelsea, and a successful appeal of Queens conviction in a 1974 attempted rape and murder, Mr. Williams fled the New York area before prosecutors in Manhattan could bring another criminal case against him, Mr. Morgenthau said.

With warrants out for his arrest, Mr. Williams eluded law-enforcement officials until last year, when he was arrested after attempting to purchase a shotgun in Georgia’s DeKalb County. A background check disclosed the outstanding warrants, and Mr. Williams was arrested. He was extradited to New York in November.

His attorney, Michael Rubin, did not return calls yesterday.

Looking for physical evidence to retry Mr. Williams for the 1973 rape, Manhattan prosecutors said yesterday, they stumbled on a pair of green cotton underwear that belonged to the victim, who was raped and robbed after a man entered her Chelsea apartment through an unlocked window and held a knife to her throat, creating cut marks on her neck.

The underwear had been collected as evidence and placed in one of thousands of legal folders kept at a warehouse used by the Manhattan district attorney to store criminal evidence. Because sophisticated DNA testing was unavailable to prosecutors then, the underwear was useless at Mr. Williams’s trial and sat in the warehouse for three decades.

Testing the underwear recently with DNA technology, forensic experts found traces of semen enmeshed in the cotton fibers. The genetic code contained in the semen sample was subsequently matched to Mr. Williams, according to the authorities.

By uploading the genetic code to a national DNA databank used by law enforcement, according to the D.A.’s office, prosecutors linked Mr. Williams to nine rapes and robberies in Montgomery County, Md.

While technology at the time required large samples of biological evidence such as semen to obtain accurate DNA codes, law-enforcement officials in Montgomery County said yesterday that the similar nature of the crimes suggested to them that the man responsible also committed several other rapes and robberies.

In the suburbs surrounding Washington, such as Silver Spring and Chevy Chase, in the late 1980s and early 1990s, crimes committed by a serial rapist and burglar created an air of fear for residents and frustration for law-enforcement officials. The authorities said more than 25,000 hours were devoted to looking for a suspect. Dozens of surveillance teams were deployed. More than 3,000 potential suspects were interviewed.

Still, local investigators uncovered no significant clues to a suspect’s identity – until now.

One of the lead detectives in those cases, Lieutenant Ronald Raum, of the Montgomery County Police Department, told The New York Sun yesterday that investigators had everything they needed to convict Mr. Williams at the time, except his name.

The crimes were eerily similar, Mr. Raum said. Wearing a ski mask to shield his face, the suspect would most often enter a home through an opened window, always at night and always when the victim’s husband or boyfriend was not at home. Using a cutting instrument, such as a box cutter or a folding knife, he would threaten the woman, then rape and rob her.

The suspect had a noticeably bad body odor though he wore cologne, police said at the time. He used different materials to cover the faces of the victims, to shield his identity.

He also used similar phrases during the attacks, Montgomery County police officials reported at the time. “We’re just two strangers in the night” was one such phrase. “I’ve been watching you from afar” was another. A third: “Do you want a big ‘C’ carved in your face?”

That “C,” law-enforcement officials said yesterday, might now be considered a veiled reference to Mr. Williams’s first name.

While Mr. Williams has not been criminally charged with any of the rapes in the Washington area, Mr. Morgenthau said yesterday that prosecutors in his office were working with officials of Montgomery County to secure additional indictments.

Mr. Morgenthau also said that, more than three decades ago, he met personally with the victim of the 1973 rape in Chelsea, to discuss laws and methods used to convict rapists. Yesterday, he praised the advances in forensic technology that allowed prosecutors to break open such a cold case.

“Without the underwear, without the DNA, without the warehouse, without the data bank, without it all, there would have been no case,” Mr. Morgenthau said. “This should send a chill through defendants, to know that you can still test for DNA after 32 years.”

Mr. Williams is being held without bail at Rikers Island.