A 49-year-old man serving time in a Virginia prison has been charged with the rape of a woman in Waldorf 20 years ago, the Charles County Sheriff's Office announced last week.

Norman Bruce Derr was indicted Sept. 27 and charged with first-degree rape in the Dec. 9, 1984, incident.

Authorities said "persistence and technology" led to the closure of the nearly two-decades-old sexual assault investigation.

The victim, who was 29 and a real estate agent at the time, was securing a model home in the White Oak subdivision in Waldorf about 6 p.m. after a Sunday open house, according to the sheriff's office.

As she locked the doors and prepared to leave, a man wearing a mask emerged from one of the rooms with a knife. He forced the woman to another location in the house, raped her at knife point and fled.

The Charles County Sheriff's Office responded and conducted the investigation. Among the evidence collected was DNA.

The suspect, however, was not identified, and the case remained unsolved. Over the years, employees of the Forensic Science Unit within the sheriff's office had periodically submitted the DNA evidence to the Combined DNA Index System, a databank maintained by the FBI.

Then in April, the FBI notified the Charles County forensic unit that the DNA it had submitted matched Derr's.

Detectives reopened the case. Information on the DNA match and the earlier investigation was forwarded to the Charles County state's attorney's office, where prosecutors sought and obtained the indictment of Derr, the sheriff's office said.

Derr is incarcerated at the Nottoway detention center in Burkeville, Va.

He is awaiting court proceedings in Charles County about the indictment, according to the sheriff's office.

"I hope by indicting this man and finally being able to hold him responsible for the horrible crime he committed against the victim, it will bring her some closure and a better sense of security," Charles County Sheriff Frederick E. Davis (R) said.

"I am proud of the employees of the Forensic Science Unit and our detectives for continuously following up with this case," Davis said. "No matter how long an investigation takes, they are not satisfied until the criminal has been held accountable and they've fulfilled their duty to the victim."