BROCKTON, Mass. — Cindy Tangstrom knew she had an uncle who died in the attack on Pearl Harbor. But it was something that never came up at family gatherings. It was something that shattered her father.

“I think it really hurt that he lost his brother,” Tangstrom said.

But the past returned suddenly – and unexpectedly – last month when she heard from a woman named Laura Daley. I have something for you, Daley said. Something that belongs to your family.

It was the Purple Heart awarded posthumously to Sailor John Russell Johnson. That symbol of sacrifice had gone missing for decades, until Daley and her teenage daughter, Kinsey Lonergan, embarked on a search to unite the medal with Johnson’s relatives.

“I couldn’t believe it,” said Tangstrom, 69, Johnson’s niece. “It’s been so many years. I had no idea.”

Johnson had been dead more than 70 years when Lonergan, in the summer of 2013, was helping her father clean a desk at Veterans of Foreign War Post 1046 in Brockton. Daniel Lonergan was the post commander at the time.

That is when Lonergan, who was still in high school, found the medal, encased in a blue box with Johnson’s name engraved on the back.

The discovery launched the younger Lonergan, 18, and Daley on a quest to return the Purple Heart to relatives of Johnson, who lived in Brockton before he died aboard the USS Arizona on Dec. 7, 1941.

On Monday, the 74th anniversary of the Japanese attack on the United States, the medal is expected to be back in the hands of Johnson’s family.

“It’s a Purple Heart,” Lonergan said Thursday by phone. “You want to get it back to his family. You don’t want it to just sit in a drawer.”

Tracking down Johnson’s survivors proved to be a challenge. His was a common last name, and he had no children when he died at age 23.

Lonergan and Daley said they pored over records at Brockton City Hall and conducted research online to identify living relatives.

Sometimes, they hit roadblocks, setting their research aside.

Over time, Daley said they learned Johnson had graduated from Brockton High School and joined the U.S. Navy in 1937. He married Eleanor Dalton, and lived with her and her parents on Belmont Street in Brockton.

The mother-daughter team identified Johnson’s parents and siblings, but the trail started to go cold as they tried to track down his nieces and nephews. Records showed they had moved away from Brockton as adults, Daley said.

Where they went was a mystery, she said, until they turned to the website www.togetherweserved.com.

The site is dedicated to military heritage and gets considerable traffic from researchers, said Diane Short, administrator for togetherweserved.com. After details about Johnson and his Purple Heart were published at the site, a reader found an obituary for one of the slain sailor’s nephews.

The obituary led to two nieces and a grand-niece.

Tangstrom’s brother, Francis David Johnson, 71, said he visited the USS Arizona Memorial in 1964 while stationed in Hawaii with the U.S. Marine Corps. John Russell Johnson remains entombed on the battleship, his nephew said.

“He died before I was born. I never met him,” said Johnson, who spent 20 years in the Marines and lives in Plymouth. “My family never talked about it. It was just a non-subject. If you tried to bring anything up, they ignored it.”

Johnson said he was told one detail about his uncle’s doomed time on the USS Arizona: The sailor had not been scheduled for duty the day of the attack. But because he had already finished his Christmas shopping, he agreed to switch shifts.

“Then the Japanese bombed Pearl Harbor and killed him,” Johnson said.

The Purple Heart and accompanying citation are scheduled to be presented to the Johnson family during a ceremony Monday at the Brockton VFW post where the medal was found.

Daley said she believes Johnson’s Purple Heart wound up at the VFW post through Johnson’s widow, who remarried and moved out of state after her husband’s death. She has since died.

Tangstrom said she’s grateful the mother and daughter found the medal – and Johnson’s family.

“That was nice of them to keep checking until they found us,” she said. “It’s a big honor.”