Marlon M. Alexander, 39, of Germantown, Md. (Montgomery County police)

In clear script, the 87-year-old woman wrote a poem about being raped. She titled it “Hell.”

“I did not think I believed in you, the opposite of heaven, love, compassion, caring. But now I do,” she wrote. “I have experienced Hell in the form of a man, an intruder . . . a dehumanized being.”

Her words, written seven years ago, described an assailant whom police in Montgomery County, Md., said Friday they finally have in custody.

That woman, who has since died, never got to see justice. But police have also charged the man with twice raping another older woman around the same time. That woman was 68.

“I’m real glad they got him,” she said in an interview Friday from a senior-living center in Southern Maryland.

She said she’d be willing to travel to Montgomery County and testify at a trial. But someone would have to pay for her transport, she said, which isn’t easy. She spends most of her days confined to a large gurney chair because of spinal stenosis.

“He just had a thing for older ladies,” she said. “I think he lived with one who was controlling to him, and he took it out on us.”

The suspect, Marlon M. Alexander, 39, of Germantown, is due to make his first court appearance Monday. The crack in the long-running cases, police say, came through an increasingly common law enforcement tactic: comparing the DNA left by an unknown suspect at a crime scene with known DNA profiles compiled from around the country by the burgeoning ancestry research industry.

It’s not that authorities are finding exact matches. It’s that the comparisons are leading them to biological relatives of suspects, in some cases fairly distant cousins. But from there, using traditional family-tree research tools like obituaries and census records, detectives and genetics experts have been able to zero in on suspects.

“We’re going to see more of this,” said Capt. Mike Wahl, commander of Montgomery’s Major Crimes Division.

For years, the rapes of the older women in Montgomery had gone unsolved.

The first, according to police, occurred about 3 a.m. on June 19, 2010. A man went through the unlocked front door of a condominium in Germantown, confronted a 68-year-old woman in her bed, held an unknown weapon to her head and raped her. He left behind his DNA and fled.

Two months later, police said, the same suspect — armed with a pair of scissors — crawled through a window at 4:15 a.m. at a senior living center in Germantown. He raped a woman, 86 at the time, hit her several times and fled. He left DNA at the scene.

It was five months later, on Jan. 5, 2011, that the rapist returned to the first woman’s condominium. This time, at 12:45 a.m., he went through a window, confronted her in her bed, raped her and again left behind DNA.

The attacks rattled both women.

“You made the choice to prey on elderly women,” the older victim wrote in her poem, “knowing they could not defend themselves.”

As the years went by, she eventually died.

The other victim, now 77, has since moved to the senior-living center in La Plata. “They can do so much more with DNA today,” she said.

Her ailing spinal cord limits her ability to walk. “I’m in good health otherwise,” she said, going on to talk about the deer, birds, butterflies and “one big, fat roly-poly groundhog” she spots outside her window. “The best thing here is the view.”

After Alexander’s arrest, police officials Friday spoke about the DNA matching and genealogical research that led to his apprehension. One twist in this case: It wasn’t just detectives, the crime lab and genetic companies that did the work. A longtime patrol officer who has developed an interest in genealogy pitched in.

About a week ago, Officer Steve Smugeresky, known as “Smugs” since he was in the third grade, stopped by the department’s cold-case unit to speak with investigators about their progress in the case. They told him that Parabon NanoLabs, a DNA technology company in Reston, Va., had provided the names of two people who appeared to share enough DNA to possibly be relatives.

But they were just names, with zero connection to Montgomery County and no obvious family connection to the county.

“Give me a few days,” the officer said, “and I’ll have a look.”

Smugeresky used Census Bureau and obituary information to go backward in time from the two names — finding their parents and grandparents, and then siblings. Then he headed in the other direction, learning the identities of the siblings’ children and the names of their children’s children. He identified six family “prongs,” eventually finding names that had connections to Montgomery County.

Detectives then worked their way to Alexander, got a DNA sample from him and compared it with the DNA left at the rape scenes. It matched, they said. Alexander was arrested Thursday at a Germantown grocery store.

Detectives also charged him with attempted first-degree sex offense in a 2007 break-in, alleging that he entered the apartment of a 25-year-old woman and threatened to shoot her if she did not perform a sex act. She fought and he fled, according to police.

Detectives “believe it is possible that Alexander committed additional sexual assaults and continue to examine similar unsolved rapes that have occurred in the county,” a police spokesman said Friday.