WASHINGTON, Sept. 5 — In his final years in prison, Kirk Bloodsworth had a passing acquaintance with a fellow inmate, Kimberly Shay Ruffner. Mr. Bloodsworth, a prison librarian, delivered books to Mr. Ruffner. Sometimes they lifted weights together. But Mr. Bloodsworth said Mr. Ruffner seemed to behave "kind of peculiar" when they were together.

Mr. Bloodsworth may now know the reason why. This morning, the police in Baltimore County charged Mr. Ruffner in the murder and rape of a 9-year-old in 1984, Dawn Hamilton, the very crime that Mr. Bloodsworth was serving time for when he met Mr. Ruffner.

"I'm so happy," said Mr. Bloodsworth, 43, a fisherman from Cambridge, Md. "This tells the world that I'm innocent."

The charges against Mr. Ruffner open a new chapter in a case that has become a prime example of the two-edged nature of DNA testing: not only as a means of clearing the wrongly accused, but also of identifying new suspects in cold cases.

In 1993, Mr. Bloodsworth became the first person in the nation convicted in a death penalty case to be exonerated through DNA testing, which eliminated him as a source of semen stains on the girl's underpants. He had served nine years in prison, including two on death row, when he was released by a judge and pardoned by the governor.

Last spring, a Baltimore County forensic biologist who was studying evidence from the case found stains on a sheet that had not been analyzed, a spokesman for the Police Department said. Investigators conducted DNA tests on the stains and ran the results through a national database last month. Mr. Ruffner's name popped up.

The police did not have to go far to charge Mr. Ruffner. He was still serving time for attempted rape and attempted murder in the prison in Baltimore where he had met Mr. Bloodsworth.

Defense lawyers say they hope the dual use of DNA evidence in the case will reduce resistance among prosecutors to allow prisoners to challenge convictions with DNA tests. They say the case demonstrates that DNA can not only prove innocence, but also pinpoint culprits.

"Maryland, and the nation, would be remiss if we did not learn from today's news," said Peter Loge, director of the Criminal Justice Reform Education Fund, which has championed Mr. Bloodsworth's case.

The new charges were filed at a crucial time in a debate in Florida over a law from 2001 that will soon bar prisoners from seeking DNA testing for old cases. The law set Oct. 1 as the deadline for such requests. It also allows the destruction of DNA evidence, except in death penalty cases.

Defense lawyers contend that the Hamilton case clearly shows how DNA can be used to reopen cold cases. If a law like Florida's had been in effect in Maryland 12 years ago, they say, Mr. Bloodsworth would still be behind bars.

"This should be a cautionary tale to those in Florida who are insisting on this deadline for destroying biological evidence," said Barry Scheck, co-director of the Innocence Project at the Benjamin L. Cardozo Law School in Manhattan. "It's a law enforcement calamity."

Many prosecutors contend that DNA testing, though reliable, is not a sure-fire way to prove innocence where there is other evidence of guilt. DNA testing, they say, should be seen as only one piece of a bigger evidentiary puzzle.

"I don't know of many people in my business who don't see the accuracy of DNA testing," George W. Clarke, deputy district attorney for San Diego County, said. "It's more the significance of those results. Those questions aren't about to go away."

For Mr. Bloodsworth, today's developments had a far more personal significance. Though he was pardoned a decade ago, he said some people had continued to view him as a child murderer. Worse, he feared that prosecutors remained convinced of his guilt and might try to bring new charges against him.

So after the assistant state's attorney who had prosecuted him, Ann Brobst, called him on Thursday night to ask for a meeting, Mr. Bloodsworth and his wife could not sleep.

"We were prepared for anything," Mr. Bloodsworth's lawyer, Deborah Crandall, said.

Instead, in a meeting this morning at a Burger King, Ms. Brobst told Mr. Bloodsworth of the DNA evidence against Mr. Ruffner and apologized for wrongly prosecuting him. Mr. Bloodsworth, who has become an outspoken advocate for reforming federal death penalty laws, said he cried and then hugged Ms. Brobst.

In an interview, the state's attorney for Baltimore County, Sandra A. O'Connor, said that the police and prosecutors had acted responsibly in the case, but that DNA technology did not exist at the time of Mr. Bloodsworth's trial. What did exist were the statements of five witnesses who said they saw him with the girl on the day she was killed.

"Obviously," Ms. O'connor said, "the system failed in the case of Mr. Bloodsworth."