Whoever he is — if he’s still alive — he attacked at least seven and probably nine women in Montgomery County and Georgetown in the 1990s. Eight were raped, including a 28-year-old biochemist he beat to death with a slab of rock. The other victim fought off a rape attempt until the assailant cracked her skull with a radio and fled.

Detectives have his DNA. But they don’t know his name.

“The Potomac River Rapist,” they’ve dubbed him, because most of the attacks took place in lower Montgomery in homes not far from the river. The last known assault — the only fatal one — also happened near the river, in Georgetown on a summer night in 1998. Investigators said they have no clue what became of the rapist after that.

“Just horrible, despicable acts against so many victims and their families,” said Montgomery Assistant Police Chief Russell E. Hamill. “It’s time to bring this matter to closure. We need the public’s assistance. . . . Somebody knows something out there. . . . Please, search your hearts, search your memories.”

And visit a new Web site, fbi.gov/potomacriverrapist.

A police composite of the person the Montgomery County and D.C. police departments want to interview in connection with seven sexual assaults and one attempted sexual assault between May 6, 1991 and Nov. 14, 1997. (Courtesy of FBI/COURTESY OF FBI)

Inspired by a digital public-information blitz that led to an arrest in the notorious “East Coast rapist” case in March, Hamill and other officials gathered Thursday to announce a similar campaign. It is probably a harbinger of more such efforts as police agencies increasingly use new media to hunt elusive suspects.

The Web site, launched at 9 a.m. Thursday, includes newscastlike video accounts of the nine cases, a photo gallery, a podcast about the rapes, a timeline of the assaults with a map of where they occurred, reward information, phone numbers to call with tips and a sketch of a man being sought for questioning.

The sketch and reward offer also began appearing Thursday on digital billboards along highways, in bus shelters and elsewhere in the District and 15 Eastern states.

In the East Coast rapist investigation, police arrested 39-year-old Aaron H. Thomas within days of initiating a digital media effort. Authorities said the key tip came from someone who saw news coverage of the campaign, which sought an attacker suspected in 14 sexual assaults in Maryland, Virginia, Connecticut and Rhode Island from 1997 to 2009.

“This is something we’ve tried, and has worked, four or five times across the country in the past year,” said Lindsay Godwin, a spokeswoman for the FBI’s Washington field office. She said the effort to identify the Potomac River rapist also will employ iTunes podcasts as well as YouTube, Facebook, Twitter and other social media.

“We want to be able to use everything that the public has at their fingertips,” Godwin said. “So if they’re bringing it up on their iPads, their BlackBerrys, their ­iPhones, and they can easily get not only images but video and podcasts — anything the public uses on a daily basis — we want to be in front of them.”

The Montgomery attacks attributed to the Potomac River rapist began May 6, 1991, and continued until Nov. 14, 1997. The victims ranged from an 18-year-old babysitter in Germantown to a 41-year-old woman living in downtown Bethesda. Other assaults took place in Gaithersburg, North Potomac, Rockville and Silver Spring.

All the Montgomery attacks occurred inside homes. Among other similarities, in each case, the assailant threw a blanket over the woman’s head. “None of our victims ever really got a look at the guy’s face,” said Montgomery police Detective Joe Mudano.

The only victim who wasn’t raped, a 34-year-old live-in housekeeper, put up a spirited fight, Mudano said. “That’s when he took a big boombox and started smashing her in the head with it, fractured her skull and then took off.”

By 2006, with advancements in DNA technology, officials said, they determined that the same man committed six of the eight attacks. They said no DNA evidence is available in the first rape or the assault on the housekeeper, but the method of attack in all eight cases was identical.

The last assault attributed to the rapist was much different. And it was fatal.

Christine Mirzayan, 28, who had recently moved to the District from San Francisco, was dragged into woods off Canal Road in Georgetown, then raped and beaten on the head with a 73-pound rock.

Earlier that evening, Aug. 1, 1998, Mirzayan, a newlywed, had joined friends for a barbecue. She held a doctorate in biochemistry and was in Washington that summer for a fellowship at the National Academy of Sciences. She was walking to her Georgetown apartment when she was attacked, investigators said.

One witness heard her scream, police said. Another saw a man leaving the woods moments later. The sketch appearing on digital billboards from Maine to Florida is based on a facial description of the man seen that night: an African American who looked to be in his late 20s or early 30s and to have close-cropped hair.

Officials have not publicly declared the man a suspect. He is being sought only for questioning. But he would be questioned about all nine assaults, they said, because DNA left by Mirzayan’s killer has been definitively matched to the DNA in the six Montgomery cases.

Did the rapist stop in 1998? Since Mirzayan’s slaying, “I’m not aware of another case that has these hallmarks,” said Ronald C. Machen Jr., the U.S. attorney for the District. “But people often, or could, change their modus operandi.”

Maybe he’s dead or in prison. “It could be that our offender went overseas and is in a foreign country committing similar acts,” said Ronald T. Hosko, head of the criminal division in the FBI’s Washington field office. Like other investigators, Hosko said that if the rapist is alive and free, he is probably preying on women.

As for the various databases of criminals’ DNA across the country, Hamill said, “We’ve exhausted those to the extent that we can,” searching for a match. “Frankly, he could be in prison on a charge that wouldn’t require a DNA sample to be submitted. He could be in a state where they don’t load up their DNA samples the way we do.”

For now, the official said, the digital media blitz is the best hope.

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