The 82-year-old woman was walking her dog in the alley behind her Minneapolis home in November 1996 when a stranger raped and beat her. She died 12 years later, her assailant never having been caught.

Then, in 2011, DNA tests yielded a hit. A suspect, Kevin Quinton Haynes, was arrested last summer and now awaits trial. The woman’s case is one of hundreds that Minneapolis police and Hennepin County investigators and prosecutors — armed with scientific advances that now allow the most minute traces of evidence to be tested — are pursuing with new vigor.

It’s paying off: To date, investigators believe they have solved two murders and uncovered three serial rapists as the extraordinary detective work unfolds. They have obtained 33 convictions, mostly in rape cases, and over the next year prosecutors hope to file charges in another 20 cases.

Some of the previously unsolved and ­languishing cases date back decades.

“A lot of these victims think we’ve forgotten about them, but we don’t forget,” said ­Minneapolis Police Lt. Mike Martin, who heads the sex crimes unit.

The work began in 2009, after the county received a $500,000 Department of Justice grant that allowed authorities to sift through thousands of old rape and murder case files looking for evidence they could submit for DNA testing.

Searching old evidence boxes, they found and then tested nearly 600 biological samples connected to unsolved cases, including hair, skin cells, blood and semen.

Forensic scientists obtained DNA profiles of 420 possible suspects from those samples, loaded them into the national DNA criminal database, and stepped back so investigators and prosecutors could methodically piece things together.

Most of these cases never made headlines. Some were random crimes of opportunity. In others, police had prime suspects but not enough ­evidence. Most involved victims on society’s fringe, said Steve Redding, senior assistant Hennepin County attorney who handled the office’s first DNA case in 1987 and oversees the cold case prosecutions.

“Many of the women who were sexually assaulted are vulnerable women,” he said. “They were on the street for mental illness, alcohol and drug problems. They were homeless.”

One crime was the 1993 slaying of a male prostitute dressed as a woman. Charges have been filed in that case, but it’s on hold as suspect Billy Daymond Bailey finishes a federal prison sentence for another offense.

In another case, prosecutors got three DNA hits for Melvin Franklin Watson, who was convicted in 2012 of raping three young women in 1993, 1997 and 1999.

“These [cases] pose huge challenges but huge rewards,” said Hennepin County Attorney Mike Freeman. “You can’t believe how grateful victims and their families are when we solve one of these.”

The most vulnerable

But in many cold cases, Redding discovered that victims could be nearly as elusive as suspects.

After reporting the crime, they often disappeared or avoided police. When authorities could not find them, cases were shelved.

Biological evidence recovered from such victims didn’t always make it to the lab. A sample might have been deemed too small or a victim’s absence made the case a lower priority.

“Frankly, that was kind of haphazard, looking back on it,” said Redding of testing decisions. “Sometimes the efforts made were good and sometimes, frankly, they were ­terrible.”

Some cases simply went cold from lack of leads or usable evidence at the time.

Today, even when authorities get a DNA match, finding a victim who is willing to testify is critical — and can be difficult because people move, names change, lives change.

“DNA helps us identify the person, it doesn’t prove the crime,” Freeman said. “We have to prove beyond a reason doubt the person did it and the crime occurred.”

Many victims — surprised, perhaps, and happy to know police are still working their case — agree to cooperate. But not all.

“You are bringing up something that they have worked on and lot of times have tried to put behind them,” Redding said. “Sometimes they have not told their husband or parents. They didn’t tell them then and they don’t want to tell them now.” If a victim refuses to cooperate, prosecutors usually will not pursue the case.

When prosecutors do ­proceed, every part of the case must be revisited. The case may have been well investigated originally, but memories fade, witnesses die or can’t be located, evidence is lost or ­misplaced. “You get a new lead now, It’s like a new case. You start all over again,” Redding said.

One advantage

There is one surprising advantage to prosecuting rape cases years later.

The victims, often characterized by defense attorneys as “young wild, careless, reckless girls,” are now women. Many have matured and changed their lives, Redding said.

“They present themselves much better than they would have back then. We use that as an argument for their ­credibility,” Redding said.

The most common defense strategy is to argue the victim actually consented. But the fact that a victim is willing to testify to an assault years later can serve as strong evidence, he said.

Closure after 17 years

In one case, the victim was 12. Her mother had kicked her out of the house for the night. She wandered into a rundown apartment building where a man held a razor blade to her throat and raped her.

The case was resurrected with a DNA match in 2011 and a suspect, Phillip Jackson, was charged. The victim traveled from out of state to testify at trial. A jury convicted Jackson of first-degree criminal sexual conduct in July 2012, and he was sentenced to nearly 14 years in prison.

In a victim impact statement submitted at the sentencing, the woman talked of how the assault changed her life.

“The hatred that I was feeling made me a very mean little girl,” she said. She described the immediate fear of pregnancy and contracting a sexually transmitted disease. She also described her lingering fear and distrust of all men.

“It’s sad that it took 17 years to get some closure to being raped, but with that being said, I am extremely grateful to everyone involved,” she said in her statement.

Police say that in many cases, the suspects have long rap sheets. That’s how the DNA got into the system in the first place. “At least half of the cold-case hits are to people that are incarcerated. But some of these guys fly under the radar,” Martin said.

Haynes, accused of raping the 82-year-old woman, has a lengthy criminal history. That’s why police and prosecutors chose to move forward with the case even after the victim’s death. “This one really offended us,” Freeman said.

The most high-profile case to be resolved was the oldest: the 1980 slaying of Mary C. Steinhart, a successful young woman found dead in her Minneapolis apartment. Robert Skogstad was arrested in Kansas in 2012 and pleaded guilty to second-degree murder last fall.

There are still 240 unidentified DNA samples harvested during the two-year sweep that remain in the national database. They are compared to every new offender profile entered into the system. Last week, investigators were notified of two more new DNA matches in two old rapes. DNA found on a sock in a 1990s stranger rape matched an offender in Omaha and DNA found in another case matched DNA found in a ­Florida assault.

“It’s amazing to me we are still getting cold case hits today,” said Minneapolis Police Lt. Martin.

Redding, who has prosecuted sex crime for much of his career, said there is some satisfaction at nabbing people years later.

“People who have been victimized, they need a voice. We try to give them a voice,” ­Redding said.