All three of these amateur sleuths used freely available Internet resources to help reunite the unidentified dead with their names.

Such cases are mysterious and vexing, especially in an era when identities are so inescapably public. Over the past decade, the Internet helped create a growing army of volunteers who, with nothing more than a computer and the uniquely human ability to spot similarities buried in mountains of data, are helping bring closure to families and spurring law enforcement to reopen the most frigid of cold cases.

These amateur sleuths share information gleaned from newspapers and other public sources on websites created for just that purpose, and they connect with one another, but not necessarily face-to-face. Participants across the country range from Web-surfing college students to professionals like Dan Brady, a software sales rep from Holliston, to stay-at-home moms who are true-crime fiends. “You meet other people, and develop relationships with some you’ll never meet in person,” Greenwood said. Brady said he regularly corresponds with a dozen Web sleuths he wouldn’t recognize on the street.

There are plenty of cases to investigate in Massachusetts. Many have heard of the Lady of the Dunes, the nameless young woman who was found with her hands cut off and her head bashed in on a Provincetown beach in 1974. There are at least 20 other local Jane and John Does: the disembodied cranium of a young woman found 6 miles off the coast of Marshfield in 1976; a 20- or 30-year-old man with two bullet wounds to the head and a distinctive metal medallion around his neck, found in a wooded section of Burlington in 1975; a middle-age Hispanic woman, wearing a red-and-yellow hoodie, prescription eyeglasses, and silver jewelry, found near Tolland State Forest in 1995; and a man with a tattoo of a broken heart and a copy of the Bible, whose body was floating face down in the Charles River in Waltham last August.

Circumstances unexpectedly thrust Greenwood into the world of the missing. In June 2000, when 16-year-old Molly Anne Bish vanished from a lifeguard post at a pond in sleepy Warren, Greenwood, mother of one of Bish’s classmates, logged onto the nascent Internet to spread word of the search. Greenwood stumbled upon the Doe Network, a website populated with photos of the missing as well as dozens of images of clay busts and other artist reconstructions of unidentified human remains: a kind of Facebook of the dead.

Brady made his way to the Doe Network when he spied an item in the Globe about a perplexing 1957 Philadelphia case known as the Boy in the Box, a 4- to 6-year-old murder victim dumped in a field in a cardboard box. “It seemed like, ‘Wow, somebody should have claimed that boy,’ ” he said. “It’s a little haunting.”

In 2007, a National Institute of Justice (NIJ) census of coroners, medical examiners, and law enforcement agencies estimated that there may be as many as 40,000 unidentified individuals — more than a sold-out Fenway Park — stowed in the back rooms of morgues and buried in unmarked graves across the country. The federal agency, a research and development arm of the Justice Department, called the little-known reality “the nation’s silent mass disaster.”

The Doe Network, whose website launched in 1999, and its spin-offs have swelled over the past decade to include hundreds, perhaps thousands, of volunteers who post details about the missing and unidentified culled from public records and the media. Some use official medical examiner and coroner websites such as Las Vegas Unidentified and the Florida Unidentified Decedent Database to ferret out possible matches, even though it means perusing disturbing post-mortem photos, computer-generated color portraits, cartoon-like illustrations, and distorted clay dummies sporting wigs, like something out of a beautician’s academy for the hopeless.

Greenwood, who once painted portraits, found she had a knack for “looking at reconstructions and photos and comparing the proportions of the ears, eyes, or length of the nose. I was able to look at different things and say, ‘This might be a possible match.’ ”