ROME — The body of Yara Gambirasio, a 13-year-old student, was found on a chilly day in a quiet northern city in Lombardy, Italy, in February 2011, three months after she was reported missing following a gymnastics practice. The autopsy showed she had been beaten but not sexually abused, stabbed several times and left to die in a field less than 10 miles from her home.

The autopsy also revealed stains on the young girl’s frayed clothing: a liquid that contained the DNA of a person whom investigators called “Unknown Male No. 1.”

In the absence of other evidence, Italian investigators embarked on the country’s largest DNA dragnet, taking genetic samples from nearly 22,000 people.

Finally, last month, they said they had found their suspect, Massimo Giuseppe Bossetti, a 43-year-old father of three from Lombardy.

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But what some praised as a triumph of modern science and 21st-century sleuthing has also set off a debate about the risks of privacy violations and the darkened corners of the past they can expose after the DNA testing also revealed something unknown even to the suspect’s family: that he was the newly discovered son of a man who had died in 1999.

Geneticists arrived at this conclusion after narrowing the DNA search to one local family and a long-dead bus driver whose known sons did not match the genetic sample found on the slain girl’s body. The dead man’s DNA was first lifted from a stamp he had licked. Later, he was exhumed to carry out further tests.

When a match was made last month, and Bossetti was arrested, the ensuing media frenzy was set off in part by the details of what one newspaper described as a “genetic soap opera.”

Recent confidences on the part of the residents of small towns in Lombardy about a long-ago — and until now secret — affair eventually led investigators to the mother of “Unknown Male No. 1,” who presumably cuckolded her husband with a local acquaintance in the late 1960s.

Thousands tested

Italy’s privacy watchdog eventually berated the media for leaking news of the alleged affair and for pursuing comment from its unsuspecting protagonists, notably the cuckolded husband and wife, and the suspect’s legitimate and illegitimate siblings.

A sharply worded statement by Italy’s privacy authority called on media outlets to use “maximum respect” in reporting personal details incidental to the case of the murdered girl. “Not even the public interest legitimizes the media frenzy over intimate personal details such as to create irreparable damage to family life and personal relations,” Antonello Soro, the chief of Italy’s Data Protection agency, said in a statement.

Investigators had initially concentrated their efforts on tracking down people whose mobile phones had been used near where the girl was abducted and killed. The families of classmates were tested, as were regulars at a nearby discothèque and the members of the sports center where Gambirasio had taken gym classes.

Investigators said they could have eventually tested all 120,000 phone owners intercepted in the area that night. “But I don’t know if we would have gotten them all,” said Letizia Ruggeri, the lead prosecutor in the case.

Despite the sheer number of people tracked down for DNA testing, the investigation raised few eyebrows in Italy. A court order can be issued in case someone refuses to be tested, “but I never needed one because everyone submitted to the test voluntarily,” Ruggeri said.

The previously unknown familial ties and their public exposure are the unfortunate “collateral effects” of a complicated case, said Carlo Previderé, a forensic geneticist at the University of Pavia, where hundreds of DNA samples were tested. The testing eventually identified the birth mother.

Previderé’s lab positively identified Bossetti after testing DNA material taken during a breath analyzer test staged by investigators, who pretended to stop him by chance.

By most media accounts, the case is effectively closed.

DNA “very revealing”

Valerio Spigarelli, president of the national criminal lawyer’s body, is among those who have challenged the public dissemination of genetic information gathered through such vast DNA screening.

“When something leads to very personal rights, like paternity or maternity, perhaps the results could have been guarded a little more jealously,” he said.

Prosecutors say that while the DNA match is “very revealing,” other evidence consolidates their case, including the suspect’s presence in the area on the night of the murder. Furthermore, the suspect’s genetic material was found on the young girl’s underwear and leggings, “a sensitive spot, next to a cut” and its presence in that spot would be difficult to explain otherwise, said Ruggeri, the prosecutor.

“DNA doesn’t fly,” said Luciano Garofano, the retired commander of the military police’s leading forensic department, based in Parma.

Bossetti insists he does not know how his DNA got onto Yara’s body. He spoke to prosecutors this month, “answering all the questions put to him, and explaining some elements,” one of his lawyers, Claudio Salvagni, said.

Yet even geneticists certain of the DNA match balk when it comes to definitively pinning the murder on the “Unknown Male No. 1.”

There is no doubt, said Giuseppe Novelli, an acclaimed geneticist and chancellor of the University of Rome Tor Vergata, whose department worked on the case, that the DNA traces found on the young girl belongs to the match. There is no doubt, either, “that he is the illegitimate son of a dead man,” he said.

“But whether he is the murderer, I don’t know,” Novelli said. “Now we have to decide how it got on Yara’s body.”