NEW YORK — On a winter's night in early 2004, after a late visit at his parents' house near Cleveland, Javier Reveron called his mother to let her know he had driven home safely. Then he vanished.

Evidence would point to New York. A plane ticket was purchased and used. A car parked near La Guardia Airport was broken into, and some of Reveron's things were found: his wallet, a driver's license, business cards and a banana peel. There would be sightings, most likely false, in places like Queens and Ohio. Then the trail went dry.

But about six years after his disappearance, the New York City medical examiner's office discovered what happened to Reveron.

The office is undertaking an ambitious effort to identify the nameless dead in the city's potter's field, seeking to capitalize on the expertise that it gained over the past decade identifying remains from the World Trade Center attack.

Through old-time detective work and newer DNA technology, the office established that Reveron, 27, had drowned not long after arriving in New York in early 2004, and that his body was buried on Hart Island, home to the potter's field, the graveyard of the poor, the unclaimed and, in rare cases, the unidentified.

John Does exhumed

Some 980 unidentified bodies have been found in the city, or its waterways, since 1990. After a month or more in one of the city's morgues, the bodies are generally sent to be buried in the same trench graves on Hart Island as the indigents.

But now, as the medical examiner's office conducts a systematic review of its old cases, the office is not only reopening dormant case files; it is also opening old graves.

Since 2010, the city has exhumed 54 bodies from the potter's field for further study. So far, the effort has led to about 50 identifications, mostly through DNA evidence.

In the case of Reveron, his parents, Rigoberto and Judith, came to New York to try to find their son. They handed out fliers and visited store owners who might have seen him. When friends visited New York, the Reverons would give them more fliers to distribute.

"All to no avail," Rigoberto Reveron said via telephone from his home in Lorain, Ohio. "We didn't know he had already been buried."

In March 2010, Rigoberto Reveron filled out an electronic form concerning missing and unidentified people. Within a few days, he got a call from Ben Figura, the director of identification at the medical examiner's office, who said, "I personally want to get involved and help you find him."

The medical examiner's office got critical biographical information about Reveron's son: He was a high-school and college wrestler who was diagnosed as bipolar after college. He had an appendectomy, had donated a kidney to his older brother, and had the scars from both surgeries.

That information, Reveron said, matched "a John Doe buried on Hart Island." The match was then confirmed by DNA, using an autopsy sample and DNA taken from the parents.

"In April 6, 2010, our chief of police and pastor of our church came looking for my wife and I to tell us a positive match had been made," Reveron said. "Six years later."

'Very depressing'

The medical examiner's office declined to discuss any case in which an identification had been made, citing privacy concerns. But the cases, according to a source familiar with the identification process, have also included Sean Wheeler, a professor at John Jay College of Criminal Justice who went missing in late 2003 after a car accident on the Henry Hudson Parkway.

Although his body was found about three months later, floating in the Hudson River by a ferry captain, it was not until 2010 that a match was made.

The confirmation of Wheeler's death "was very depressing," said an aunt, Kimberly Wheeler, of Independence, Mo. "It shouldn't have taken so long if they'd put any effort into it."

Not all of these nameless dead perished unnoticed. The unknown cases include several Chinese immigrants who drowned after the Golden Venture ran aground off Queens in 1993. The episode drew extensive news coverage and shed light on the brutal conditions that some Asian immigrants endured to make it to New York.

Census of 'unknowns'

Another old case that has attracted the attention of the medical examiner's office is that of "Baby Hope." The girl, about 5 years old, has been known as Baby Hope since shortly after her body was discovered packed into a picnic cooler off the Henry Hudson Parkway in 1991.

As the subject of an active homicide investigation, her body was exhumed in 2007 from St. Raymond's Cemetery in the Bronx even before the medical examiner's office had begun looking into all of its old unknown cases. But at the time, biologists were unable to extract any DNA because her bones were in such poor condition.

"She had been in a cooler in the sun, and her bones were very brittle," said Sheila Estacio Dennis, an assistant director in the medical examiner's department of forensic biology.

By 2011, because of advances in the medical examiner's office in the DNA extraction process, Dennis said, the office was able to obtain a full DNA profile. It did not yield a match to existing DNA databases from convicted felons or from active missing person cases in which DNA samples had been collected.

Investigators believe her parents, or one of them, most likely murdered her, as no one has come forward to identify the girl as his or her child.

In reviewing hundreds of old cases, the medical examiner's office has in effect been conducting a census of "the unknowns," as Figura calls them.

The deceased are overwhelmingly male, typically white and often homeless. They are usually found in Manhattan. The sidewalk is a common death bed.

Some 15 percent are presumed homicide victims. About 22 percent of the unknowns are pulled from the water, and investigators presume that many are suicides.

In most of the older cases, the medical examiner's office typically keeps a tissue sample from the autopsy; the samples have been used to extract DNA samples in numerous instances over the past three years. In cases without an autopsy sample, the medical examiner's office has sought to exhume the body.

Fingerprints have also led to identifications in a few cases, as improvements have been made in the sensitivity of the software that compares uploaded prints to various databases. Simply running the prints again years later has resulted in several identifications, Figura said.

Central to the medical examiner's efforts is a public database, called Namus, containing information on 7,645 missing person cases and the remains of 8,516 unidentified victims. The database has helped investigators, the relatives of missing persons and even amateur sleuths try to establish matches between the known missing persons cases and the unidentified.