COLUMBIA, S.C. -- Police solved the 1988 stabbing death of an 80-year-old South Carolina woman not just with a cutting-edge DNA test but also with nearly two years of interviews and other old-fashioned police work, authorities said.

The DNA test in 2017 only proved the suspect had been in Alice Haynsworth Ryan's car, which was stolen after she was killed in her Greenville home, Greenville Police Chief Ken Miller said.

It took dozens of interviews with the suspect, his family, friends and others involved in the case to finally lead to a murder charge, Miller said at a news conference Monday.

Brian Keith Munns, 51, was arrested in February on lesser charges, but Miller said he intentionally kept the arrest secret so detectives could do more interviews and get the best case they could in the death of the daughter of a well-known mill president and the great-aunt of the current Greenville Mayor Knox White.

One of those interviews was the key to the murder charge.

"An acquaintance of the suspect revealed detailed information the suspect described about the murder of Alice Ryan to include method of entry, injuries inflicted, area of the encounter, location of evidence and the timeline of events," Miller said.

Munns, who is a registered sex offender in South Carolina after a 2001 rape conviction, now is charged with murder, burglary and armed robbery. He was living in Americus, Georgia, when he was arrested. A message to Munns' lawyer Monday wasn't returned.

The chief did not say what may have led Munns to Ryan's home that day or if he knew her or her family. Munns lived within 2 miles (3.2 kilometres) of Ryan, and Ryan's stolen 1967 Ford Galaxie was found between the two houses with items used to attack Ryan and things stolen from her home found nearby, authorities said.

The DNA that linked Munns to Ryan was found in that car and could be tested nearly 30 years after her death. Authorities did not give more details about the nature of the evidence.

"Fortunately these detectives have done an amazing job maintaining and preserving evidence," prosecutor Walt Wilkins said.

Ryan was killed after her daughter dropped her off at home after a chemotherapy treatment and left for less than an hour to run errands. She returned to find the rear door forced open and her mother's car missing.

In April 2017, Greenville police gave the murder case file to its expanded cold case unit. The DNA test came back with a match to Munns in November 2017 and detectives started a number of interviews including with Munns, Miller said.

Ryan's son thanked the investigators who never quit looking for his mother's killer.

"The people who have been working this thing are unbelievable," Joe Ryan said. "They have gone all out, and all out for a long time. And they don't give up."

The cold case squad in Greenville has made arrests in three old cases in the past year. The first two were cracked with DNA results from family genealogy sites.