A 4-year-old boy killed in 1972 finally has been identified. But what happened to his baby brother is still a mystery.

A 4-year-old boy who was killed more than 50 years ago has finally been identified thanks to genetic genealogy -- but mysteries surrounding his murder, and what happened to his baby brother, remain.

On June 13, 1972, the little boy's body was found under a bridge in Lorton, Virginia, according to Fairfax County police. He died from blunt force trauma and the case was ruled a homicide, police said.

For over 50 years, police worked to find his name and what caused his tragic murder.

Now, he's been identified as Carl Matthew Bryant, who died weeks after his 4th birthday, Fairfax County police announced on Monday.

Authorities said the breakthrough came thanks to genetic genealogy, which uses an unknown person's DNA to trace his or her family tree. His DNA profile was obtained from just a few millimeters of hair, police said, and then genetic genealogy helped detectives track the little boy's family to Philadelphia.

Through a relative, detectives zeroed in on Vera Bryant as the mother, police said. Vera Bryant died in 1980; her body was exhumed and DNA confirmed she was Carl's mom, police said.

Detectives believe Vera Bryant and her boyfriend James Hedgepeth were involved in Carl's death, police said. Hedgepeth, who served time for murder before he started dating Vera Bryant, is also deceased, police said.

Although Carl has a name, police are now searching for his little brother.

Vera Bryant had a 6-month-old boy named James Bryant, and detectives believe the missing baby was killed around the same time as his older brother, police said.

In June 1972, Vera Bryant and James Hedgepeth traveled from Philadelphia to Hedgepeth's relatives in Middlesex County, Virginia. When the couple arrived, they didn't have her sons with them, police said. Over Thanksgiving in 1972, when the couple visited Vera Bryant's family in Philadelphia, Vera Bryant told them the children were in Virginia with Hedgepeth's family, police said. The couple never reported the boys missing, police added.

Detectives believe James Bryant's body may have also been left along the couple's route between Philadelphia and Virginia in June 1972, police said.

At a news conference on Monday, Fairfax County Police Chief Kevin Davis praised the detectives who did the "hard work" to identify Carl.

"You still knock on doors, you still talk to family members, you still talk to potential witnesses," Davis said. He also highlighted the power of genetic genealogy, which he said allows the department to "bring closure far, far more often than we ever have."

“To see the extent of that boy’s injuries and what he had suffered through, I’m happy to be here today announcing that at least we’ve identified him," detective Melissa Wallace added. "He can have his name, we can get him his name back on his gravestone and the family can have some semblance of closure or resolution."

The homicide investigation is ongoing. The Fairfax County Police Department urges anyone with information to call its Major Crimes Bureau at 703-246-7800, option 2.