Advances in analyzing small, degraded amounts of DNA and the ability to trace genetic markers that are shared by male family members allowed the Boston Police Department crime laboratory to gather convincing evidence to link Albert DeSalvo to the 1964 murder of Mary Sullivan.
Since the mid-1990s, Donald Hayes, director of the Boston Police Crime Laboratory, had realized that samples taken from the crime scene, including semen on a blanket, contained DNA that could be used to identify the killer. He simply needed the right technology to unlock that valuable information.
Early attempts to use DNA testing had provided inconclusive results in 1999 and 2001. The samples, retrieved before DNA testing had been developed as a forensic technique, had deteriorated over the years. Hayes did not want to squander what was left — samples are destroyed when their DNA is analyzed — until he was confident technology was mature enough to provide real insight.
Last year, after DNA analysis helped police crack several other cold cases, he decided the time was right. Samples from the Sullivan case were sent to two independent laboratories.
Samples taken from Sullivan’s body during her autopsy and preserved on slides contained two people’s DNA, hers and an unidentified male’s. The male portion matched DNA retrieved from semen taken from a blanket at the crime scene, leading investigators to conclude they now had the genetic profile of her killer.
However, police did not have a DNA sample from their prime suspect, DeSalvo, so they resorted to a technique that allows investigators to make matches using family members. The technique, called familial DNA searching, has become an increasingly common tool, used to identify victims of mass disasters such as Hurricane Katrina, large casualty events such as the World Trade Center bombings, and in routine police work.
“These are the same methods we use in all mass disaster cases, where if we don’t have a known reference sample from an individual — a hospital biopsy, unlaundered clothing, a toothbrush that provides the reference sample to compare other samples to — then we have to go to relatives,” said Dr. Frederick Bieber, a forensic DNA specialist at Brigham and Women’s Hospital.
Such testing relies on the fact that certain kinds of DNA are handed down from mother to child or father to son virtually unchanged. Police followed DeSalvo’s nephew, the son of his brother, and retrieved a water bottle he had been drinking from to obtain DNA that would allow them to look for a family match.
Investigators focused on the Y chromosome, which only men have, looking specifically at “short tandem repeats,” regions of DNA where a short sequence of molecules is repeated over and over. The number of repeats varies; some men have more repeats of a particular sequence than other men. Since men inherit their Y chromosome from their father, those descended from the same father will have very similar patterns of short tandem repeats.
By looking at numerous repeated regions, analysts can determine whether a DNA sample likely came from men in a particular family.
“It’s the different number of repeats that vary, individual to individual, so usually we’re looking at at least 15 locations in the DNA and determining how many number of repeats each individual has,” said Amy Jeanguenat, laboratory director at Bode Technology, based in Lorton, Va. Bode Technology is one of the firms that performed testing, but Jeanguenat said she could not comment specifically on the analysis done in the Sullivan case because of an agreement with Boston police.
The testing indicated that DeSalvo was a match and excluded 99.9 percent of the male population, District Attorney Daniel F. Conley said at a press conference.
The next step that will more conclusively determine whether DeSalvo killed Sullivan will come after his body is exhumed. Investigators will take a bone marrow sample and analyze DNA found in the nucleus of cells, Hayes said.
Several outside forensic specialists were uncertain whether, 40 years after DeSalvo’s death, the DNA will still be intact for analysis. They said it might be necessary to look at another type of DNA, found in the mitochondria, the cell’s power plants, which are outside the nucleus and are often better preserved. But mitochondrial DNA would provide a less conclusive identification.
David Foran, director of the forensic science program at Michigan State University, worked on samples taken from Sullivan’s exhumed body in the early 2000s to look for a DNA link to one of DeSalvo’s relatives.
He said he analyzed a substance in her pubic hair that was “consistent” with semen but found no DNA match to DeSalvo, but he never had access to crime scene evidence. He said the new work appears persuasive, and the analysis of DeSalvo’s remains could end one mystery.
“It certainly ends the debate about who killed Mary Sullivan,” Foran said. “There are other debates, about whether there was more than one Boston strangler. There will still be controversy.”Carolyn Y. Johnson can be reached at cjohnson@
globe.com. Follow her on Twitter @carolynyjohnson.