Nearly two decades after he became the first defendant in Massachusetts convicted of murder based on DNA evidence, Henry Juan Williams turned to the same forensic testing seeking to prove his innocence.

But DNA results, returned last month, confirmed that Williams was the perpetrator in the 1994 stabbing of 71-year-old Zachariah Johnson, Suffolk County prosecutors announced last week.

“Identifying, correcting, and preventing wrongful convictions is a fundamental part of a prosecutor’s job,’’ Suffolk District Attorney Daniel F. Conley said in a statement announcing DNA test results for Williams and two other convicts. “We have one of the most progressive policies in the nation when it comes to post-conviction testing, and we’ve never been afraid to act on that duty.’’

Williams’s conviction is among dozens of cases that have come under review since a 2012 law that lets defendants who proclaim their innocence seek new DNA testing after their trials.

About 15 people have completed the process, and tests are pending in another 10 to 15 cases, said Lisa M. Kavanaugh, director of the Innocence Program at the Committee for Public Counsel Services, which represented Williams.

In Williams’s case, a laboratory tested blood samples from the crime scene in Dorchester, the butcher knife used to kill Johnson, and a door knob from the rooming house where Williams lived, prosecutors said.

This time, the blood samples matched Williams’s DNA profile with an even greater statistical probability than evidence presented at his trial in the late 1990s, prosecutors said.

Kavanaugh declined to comment on Williams’s case. So far, no one has been cleared of a crime from testing under the 2012 law, but some case reviews have produced results that are “consistent with innocence,’’ she said.

In some instances, testing found no trace of the defendant’s DNA, although it didn’t definitively show who committed the crime, she said. In other cases, laboratories couldn’t produce a DNA profile, she said.

Kavanaugh also cited recent court orders that granted new trials for two murder convicts and a rape convict on the basis of DNA testing. The testing preceded the current law, Kavanaugh said.

“Wrongful convictions are real,’’ she said. “They have dramatic human costs.’’

Conley’s office said DNA test results were returned this year for two other men prosecuted in Suffolk County: Tyrone Clark, who was convicted of rape and other crimes, and Steven Odegard, who is imprisoned for murder.

Prosecutors contend that Clark raped a 24-year-old woman at knifepoint on June 23, 1973. During the assault, the woman took the knife from her attacker and tried to stab him, prosecutors said. The blade broke off and was never recovered.

Clark, now 61, asked for the knife handle to be tested in a legal battle that went to the state Supreme Judicial Court. The laboratory concluded that it could not produce a DNA profile, said Conley’s spokesman, Jake Wark.

“The knife handle never even penetrated the defendant’s clothing, much less his skin, and there was no chance that it would bear DNA evidence,’’ he said.

But Clark’s lawyer, Neil Raphael, said the findings don’t affirm the conviction.

“The reason why there was no DNA on the knife was most likely because the knife was 40 years old,’’ he said.

Clark maintains his innocence, said Raphael, who criticized prosecutors for fighting his request for DNA testing.

“When we moved for DNA testing, the state came back and tried to argue that he shouldn’t be granted access under the statute,’’ he said. “The whole point of the statute was to grant access to people like Mr. Clark.’’

Wark said Clark declined two previous opportunities for testing.

Odegard, 49, is serving life in prison for the murder of 20-year-old Daniel Yakovleff, who was found stabbed to death in Odegard’s home in 2008. At his trial, his lawyers suggested two other men were responsible for the killing, prosecutors said.

His defense lawyers sought DNA testing, which resulted in an examination of hairs and towels from Odegard’s apartment, the knife used to kill Yakovleff, and Yakovleff's fingernails, prosecutors said. DNA samples from two people named by defense lawyers as potential suspects were also submitted, officials said.

The results linked Odegard to the towel, and ruled out Yakovleff and the potential suspects, Wark said. DNA from the knife matched Yakovleff, officials said.

Odegard’s lawyer, Robert F. Shaw Jr. said the results were inconclusive, noting that the DNA of a third person was found on the towel. There is more evidence that should be tested, he said.

“My intention is to explore more evidence and engage in further testing,’’ he said.

Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.