They swab the cheeks of strangers and pluck hairs from corpses. They travel hundreds of miles to entice their suspects with an old photograph or sometimes a free drink. Cooperation is preferred but not necessarily required to achieve their ends.

If the amateur genealogists of the DNA era bear a certain resemblance to members of a "CSI" team, they make no apologies. Prompted by the advent of inexpensive genetic testing, they are tracing their family trees with a vengeance heretofore unknown.

"People who realize the potential of DNA," said Katherine Borges, a co-founder of the International Society of Genetic Genealogy, "will go to great lengths to get it."

Unlike paper records, which can be hard to come by and harder to verify, a genetic test can quickly and definitively tell if someone is a relative. But not all potential kin are easily parted from their DNA. Some worry about revealing family secrets. Some fear their sample could be used to pry into other areas of their lives. Some just do not want to be bothered.

Those cases inspire tactics that are turning the once-staid pursuit of genealogy, perhaps second only to gardening among American hobbies, into an extreme sport.

Derrell Teat, 63, a wastewater coordinator, recently found herself staking out a McDonald's. The man she believed was the last male descendant of her great-great-great grandfather's brother had refused to give her his DNA. So she decided to get it another way.

"I was going to take his coffee cup out of the garbage can," said Teat, who traveled to the Georgia mountains from Tampa, Fla., with her test kit. "I was willing to do whatever it took."

At one time, she might have been satisfied with a cousin's census research, which revealed that they had descended from one John B. Hodgins living in South Carolina in 1820. But a DNA test of an Oklahoma Hodgins, who was found through the phone book, confirmed they were related. Now Teat wants to identify all of John B.'s living descendants by July, when she will preside over a Hodgins family reunion.

Alas, cornered in his garage, Teat's quarry refused to listen to her pitch. Perhaps he thought she was seeking a paternity test. In any case, he did not show at his usual breakfast spot.

"It drives me nuts," Teat said. "Knowing I can get to the bottom of it, if people would just cooperate."

By next year, close to half a million people will have taken a DNA genealogy test, according to estimates from companies that provide them. The tests detect genetic markers that distinguish the descendants of an individual and reveal if two people share a recent common ancestor.

Seeking to expand their family trees, thousands of amateur family historians have begun asking people with the same last names to compare genes, even though most are total strangers. That is where the free drinks come in.