(NewsNation) — A lawyer living in New Jersey has been accused of being a serial rapist for crimes he allegedly committed in Boston 15 years ago. How did the FBI tie him to the crimes?

The same way law enforcement tied Bryan Kohberger to a quadruple homicide in Idaho — through the help of genetic genealogy testing.

The process works by taking a DNA sample and looking for matches in a public database, in this case on GEDmatch and Family Tree DNA, the two smallest.

“We’re looking for partial matches, people who share a certain percentage of their DNA with the unknown suspect; that’s typically second, third, fourth cousins and beyond,” explained CeCe Moore, chief genetic genealogist at Parabon NanoLabs. “So, if you share about 1% of your DNA with someone, you’re likely a third cousin; that means we build the family trees back to their great-great-grandparents.”

After a family tree is built, law enforcement can then identify a likely suspect from whom to collect a DNA sample for comparison to DNA found at a crime scene.

“This is really the gumshoe detective work, where you’re out conducting surveillance and you’re looking for that opportunity to grab that glass, that straw, that cigarette, whatever it might be, and protect it and get it to the lab,” former FBI Agent Jennifer Coffindaffer said.

Police did just that to arrest and charge a man they allege perpetrated multiple rapes and sexual assaults in Boston.

Matthew Nilo, of Weehawken, New Jersey, pleaded not guilty Monday to several charges, including three counts of aggravated rape, two counts of kidnapping, one count of assault with intent to rape and one count of indecent assault and battery. The charges stem from four attacks that happened in Boston’s Charlestown neighborhood from August 2007 through December 2008 — a time that authorities say Nilo lived in the city.

During a corporate event earlier this year, the FBI recovered utensils and drinking glasses the defendant used, authorities said. They obtained DNA from his glass that matched the DNA from the three rape victims and was a likely match to DNA found on a glove worn by the fourth victim, prosecutors said.

Nilo’s attorney, Joseph Cataldo, questioned the way his client’s DNA was collected.

“I do understand that the procedures used by law enforcement are somewhat suspect,” he said outside of court Monday. “It seems that they obtained DNA evidence without ever obtaining a search warrant. If that turns out to be true, that’s an issue that will be pursued vigorously.”

However, Coffindaffer said the FBI and local police typically don’t stop at just DNA evidence in making a case.

“They’re going to look to corroborate everything in terms of the dates and times, where he lived, the vehicle that was used, there was supposedly a knife used and a gun used,” Coffindaffer said. “They’re going to keep working this to corroborate all the information they currently have.”

Genetic genealogy was also used to link Kohberger to the killing of four University of Idaho students last November.

Investigators say the key in the case was a sample of DNA from the crime scene being compared to trash at the home of Kohberger’s parents in Pennsylvania. Police used DNA belonging to Kohberger’s father to show a familial link, which led to Kohberger’s arrest.

Moore previously told NewsNation she thinks there’s probably more to the story than that, but it’s likely investigative genealogy played a role.

“They don’t have to include everything in the affidavit and genetic genealogy should not be used for the basis of an arrest,” Moore said at the time.

It can be used to vet tips, like the one about the white car seen near the crime scene, she said. Police would have taken trash from the Kohberger home and used DNA from that to essentially perform a paternity test against DNA taken from a knife sheath found at the crime scene.

The two largest consumer genealogy services, Ancestry DNA and 23andMe, bar the use of DNA matching by law enforcement. If they did, Moore believes it could help solve “99%” of violent criminals who leave DNA behind.

“It would be an incredible increase in public safety, and it could save lives,” Moore said. “But I don’t really see those companies allowing it … it’s just not their business plan, their policy.”

NewsNation digital producer Stephanie Whiteside and The Associated Press contributed to this report.