She wrote countless letters, pestered senators and congressmen, traveled from her New Hampshire home to Philadelphia to search news archives, scoured faces in crowds, battled with military and state officials for records, and enlisted police and private detectives.
Every roadblock she hit, she said, only strengthened her resolve and pushed her forward.
Finally, on May 31, Pennsylvania State Police were able to identify the remains of Corriveau, found stabbed to death in Chester County, and they are now seeking the public's assistance in solving the cold case.
"He was matched through my DNA," said Cleary, 58, of Conway, N.H.
On Nov 18, 1968, the same day the 20-year-old Marine from Lawrence, Mass., disappeared from the hospital, an unidentified man was found dead alongside the Pennsylvania Turnpike near the Downingtown interchange.
He was stabbed once though the heart and covered with a Navy pea coat. He carried no identification and became known as "Bulldog John Doe" after the distinctive tattoo on his upper right arm. He was buried at Longwood Cemetery in Kennett Square.
It would be days before the family learned Corriveau was missing. Cleary recalled how her father, a deputy sheriff, called the hospital to ask if her brother would be coming home for Thanksgiving, only to be told he was AWOL. That was a red flag.
"We were pretty sure he was not alive," Cleary said. "If he was, he'd be home."
Corriveau grew up in Lawrence and was known as "Bobby Dan" to his two younger brothers, Paul and Tom, and his little sister. He called her Gigi, and he loved to tease her.
Cleary remembers Corriveau as an excellent athlete who loved to play baseball and hockey. He attended Mount St. Charles Academy, a boys' boarding school in Rhode Island.
He enlisted in the Marines at age 17 and was sent to Vietnam, where he served two tours.
He fought with the Third Battalion, Fourth Marine Division, and was wounded in action three times. He was sent home with two Bronze Stars, two Purple Hearts, and a chest full of ribbons.
When he came home, Cleary taught her brother the latest dance steps.
"He didn't want to be square," she said.
But Corriveau could not shake the flashbacks that tormented him sleeping and waking, Cleary said.
Corriveau was first treated at the Chelsea Naval Hospital in Massachusetts. He was home on leave in October 1968, then traveled to the Philadelphia Naval Hospital to receive more psychiatric treatment.
Cleary said that on Nov. 18, the hospital staff reported Corriveau, who was in a locked ward, missing at morning roll call.
"All these years ... I never stopped searching," she said.
The persistence ultimately paid off, thanks in part to advances in DNA technology that have given new life to cold cases.
In July 2009, investigators from the state missing persons unit and the Chester County District Attorney's Office decided to exhume the body at Longwood buried under the name John Doe. Investigators had no idea that the body was that of Corriveau.
A DNA profile was obtained through bone samples and entered into the national missing-persons database.
Officials suspected the man had been in the military because of his tattoos, and brought in the Naval Criminal Investigative Service Cold Case Unit and the Marines.
On Dec. 5, 2011, Cleary received an "unexpected" call from NCIS asking about her brother's scars and tattoos. She was "pretty sure" they had found him.
NCIS would later visit Cleary to obtain her DNA and confirm the match.
Cleary wasn't done fighting. She insisted that the Marines change the deserter designation. And she had to go to court to get custody of her brother's body so he could be buried next to his father and deceased brother.
"After 43 years, we were shocked to find out what happened to him," she said of the slaying. "And on the other side, we were happy that we finally knew."
Anyone with information, including anyone who served with Corriveau or who knew him from the hospital, is asked to call 610-268-5158 or e-mail RA-1968MarineDeath@pa.gov.
Contact Mari A. Schaefer at 610-892-9149, firstname.lastname@example.org or @MariSchaefer on Twitter.
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