PALM SPRINGS, Calif. — On a recent afternoon, under two giant chandeliers, some of the key people responsible for the future of crime and privacy tried to solve a murder.

“She was found in 1982 in Newark,” said the class leader, who opened the session by bursting into song. That plus the little plates of cannoli positioned on each table might have led an uninformed interloper to mistake the event for the $33-a-ticket catch a killer game across town.

But though the scenario was fictional, the players involved made it anything but a game. Seated in his deceptively low-key white shorts and hiking boots was Thomas Callaghan of the F.B.I., who was involved in overseeing Codis, law enforcement’s primary criminal DNA database over the past 25 years. He recently moved over to the F.B.I.’s rapidly expanding genetic genealogy unit. A few tables back to the right, the genealogist Barbara Rae-Venter, who helped crack the Golden State Killer case, chatted with a friend who was looking to invest big in this emerging industry. A few tables over still, Curtis Rogers, a co-founder of GEDmatch, the genealogy database that is fast becoming law enforcement’s go-to tool for solving crimes, sat checking his phone.

Diahan Southard, the instructor, offered an encouraging greeting to the 115 or so people who had signed up for her genetic genealogy workshop. “I believe anyone can learn to do this,” she said. Though some were old hands in the technique, a majority of the forensic scientists, detectives, coroners and family historians were novices, eager to learn how to identify bodies and suspects by using partial matches to cousins on genealogy sites.

The workshop, held in late September, capped a week of sessions at the 30th International Symposium on Human Identification, an annual gathering for 1,000 “rock stars of DNA” as one attendee called them. It offered a revealing, yet unconventionally performative, window into forensic genealogy, at a crucial moment in its trajectory.

At the same conference, a Department of Justice representative put forth the first ever guidelines for the technique, which burst into the forensic mainstream a year and a half ago, offering law enforcement an enticing and contentious way of identifying violent offenders.

“It’s been an extraordinary 18 months,” Ted Hunt, senior adviser to the attorney general on forensic science at the Department of Justice, told a crowd as he introduced the guidelines, which go into effect Nov. 1. Later in the buffet lines and in the hallways of the appropriately futuristic-looking convention center, attendees discussed what it all meant for crime and privacy.

In part, the policy offers law enforcement guidelines around order of operations. For example, do not upload samples to a genealogy site unless Codis, the F.B.I. criminal database of nearly 14 million people convicted or charged with serious crimes, has failed you first. It also creates DNA storage guidelines, something that put Cynthia Zeller, a chemistry professor at Towson University, at ease.

In terms of violating people’s privacy, Ms. Zeller said midway through the conference, “I think it’s less of a huge deal than when I got here.”

Ms. Zeller, who formerly worked at a state police lab, had been weighing whether the new technique, which had already helped crack more than 70 cases, was as transformative to forensics as the invention of the internet or was something more analogous to a smaller, shinier iPad.

“It’s the internet,” she ultimately concluded, given that the technique ultimately makes any bit of DNA collected from any crime scene ever potentially identifiable.

But only if it does not all implode first, Ms. Southard reminded her class on the final day of the conference. “This depends on having the support of regular people,” she said. “If you can’t convince the public that it’s safe, it’s going to go away.”

Any breakdown of trust, many attendees agreed, was most likely to involve the matter of which databases and profiles law enforcement might use in their investigations.

In a quest to find relatives or ancestry information, more than 20 million people over the past decade have shared their genetic information with genealogy databases, which have differing policies for working with law enforcement. One such company, MyHeritage, explicitly prohibits law enforcement from using its site to solve crimes. And another, GEDmatch, only allows the police to use profiles of the roughly 15 percent of people who have explicitly opted in to help solve crimes.

But the question remains: Who is responsible for making sure that customers’ wishes are respected? Asked another way: Who is supposed to make sure that if an adoptee joins a site to find her biological mother, she does not end up contributing to the arrest of her cousin for burglary?

The new federal rules apply to investigations funded by Justice Department grants. They advise investigators who want to identify suspects’ DNA to limit themselves to databases that explicitly tell their users — who may include amateur genealogists and other researchers — that law-enforcement agencies may also be using the sites. But several of the conference’s attendees, mindful of the fact that no one is checking how exactly genealogists solve their puzzles of identity, questioned how the guidelines would be enforced.

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The eight pages of new government guidelines Mr. Hunt brought to the conference focus more on another tricky, but rarely publicized, aspect of how genetic genealogy has worked so far. It has sometimes involved surreptitiously collecting DNA not just from the suspect, but from his or her relatives. (That relative could have been you; no one has been required to tell you.) These investigations require finding cousins on genealogy sites and building giant family trees. Sometimes testing the DNA of one of the people in that tree — and comparing it to the killer’s DNA to see how they are related — could help genealogists find their suspect. Sometimes investigators simply ask these relatives to take a DNA test. But in other cases they will dispatch an undercover officer to collect a discarded coffee cup.

Mr. Hunt declined to say how often this has happened, but several genealogists confirmed that it has.

The new rules require the authorities to get your “informed consent” to collect your DNA from, say, a discarded coffee cup, unless the request would compromise the integrity of an investigation. Frederick Bieber, a medical geneticist and a professor at Harvard University who also presented at the conference, called that caveat a “loophole” he would like to see eliminated.

Fundamentally, it all raises larger philosophical questions, according to the attendees. If your Yelp review helped a detective identify a thief, you probably would not expect to have to give permission for that review to be used in an investigation. Most agree that a genetic profile is quite different. But is it even helpful for people to know that a distant cousin, one they may have never known, might be a serial killer?

In the workshop, Ms. Southard attempted to get the room of future genetic genealogists to think through those questions. “If people believe their DNA is who they are, but learn that their relative is a violent criminal, then it could affect them,” she said.

She constructed a scenario to push them to consider this further. In the process of identifying the girl from Newark, a cousin named Kyle popped up as a match on a genealogy site. But his profile did not have any family tree data. “How would we try to get a tree out of this person?” she asked.

Joseph Rose, with the Mobile County Sheriff’s Office in Alabama, stared skeptically at the slip of paper on which he was supposed to draft a letter. After 20 years of being in charge of crime scenes for the police in Mobile, he had recently taken on an assignment that puts him in charge of applying genetic genealogy to cold cases. “I wouldn’t write a letter,” he said. “Writing words can’t get the tone out.”

But the two women next to him — Kathryn Johnston, a longtime genealogist ready to try working with law enforcement, and Joscelyn McBain of totheletter DNA, an Australian DNA testing company — encouraged him to come up with something.

“We’re investigating a case and we’d appreciate your assistance,” he ultimately scrawled. No one at the table seemed entirely sold on it, but they agreed it was a start.