Maureen Dunn lost her husband on Feb. 14, 1968. She did not lose him the way thousands of other American military wives lost their husbands during the Vietnam War — to death, clear and confirmed, acknowledged with a funeral and proved by a flag-draped casket. Her husband, Navy pilot Joseph P. Dunn Sr., was simply lost — missing in action — when his unarmed A-1 Skyraider was shot down over the South China Sea.

Joe was on his way from the Philippines to a U.S. aircraft carrier off the Vietnamese coast and was fast approaching a scheduled trip home for a visit. Before that last flight, he ordered flowers for his wife back in Massachusetts. The bouquet was delivered to her the day she learned that his plane had gone down. It was Valentine’s Day.

“I felt like my heart was torn out of my body,” Mrs. Dunn once told the Boston Globe, “and yet I had to keep breathing.”

For years, she was neither a wife nor a widow. That was how she described living with a loved one missing in action. It was an experience shared by the hundreds of military families Mrs. Dunn came to represent during decades of dogged advocacy in Washington and internationally.

Mrs. Dunn, 72, died May 10 at a rehabilitation center in West Roxbury, Mass., said her son, Joseph P. Dunn II. She had cancer.

Maureen Dunn with a portrait of her husband, Navy pilot Lt. Joseph Dunn who was shot down in 1968 during the Vietnam War. She died May 10 at the age of 72. (Boston Globe/via Getty Images)

Mrs. Dunn started her campaign in 1968, the year her husband disappeared, rallying a dedicated band of supporters to form the “Where Is Lt. Joe Dunn?” committee. She talked to anyone who would listen to her, said her son, from shoppers at the supermarket to audiences at local high schools.

Over time, her campaign and similar ones across the country gained momentum. In 1970, when the National League of Families of American Prisoners and Missing in Southeast Asia was formed in Washington, Mrs. Dunn was among the original members. She served over the years as a regional and national coordinator.

She helped lead demonstrations about the POW/MIA cause and met with ambassadors, presidents and other officials to lobby for support. She was credited with promoting the use of the now-iconic black-and-white POW/MIA flag, a symbol of solidarity with the missing.

Testifying before the Senate Foreign Relations Committee in 1974, she expressed her anger at what she considered the insufficient public attention to the POW/MIA issue.

“Our problem,” she said, “has been Watergated, Agnewed, Richardsoned, energy-crisised and Mideasted practically out of existence.”

Mrs. Dunn campaigned on behalf of all POW/MIA families while she doggedly sought information that might lead her to her husband. In 1995, she drew national attention when she confronted Robert S. McNamara, the former defense secretary and architect of the Vietnam War, at a presentation of his book “In Retrospect: The Tragedy and Lessons of Vietnam.”

“Mr. McNamara, you don’t know who I am,” she told him, according to a Washington Post account of the event at Harvard University. She recounted what she maintained was her husband’s story — how Joe’s plane had sent distress signals after crashing in the Chinese waters, and how McNamara and other officials determined that a rescue attempt would not be “worth it” because of problems it might cause with China.

“I’m that guy’s wife,” she told McNamara, crying. “I have waited 25 years for someone to say, ‘I am sorry.’ ”

At first, The Post reported, McNamara said he did not remember the episode. Then he said: “If I said that, I’m not just sorry. I am horrified.”

It was former secretary of state Henry Kissinger, the Boston Herald reported, who told Mrs. Dunn what he believed to be her husband’s fate. “Maureen,” he told her after a trip to China, “I feel they know what happened to Joe, but do I think you’ll ever see him walk through the door again? No.’’

In the early 1980s, Joe was declared presumed killed in action and was given a military funeral at Arlington National Cemetery. A decade later, Joe Dunn II went to China. He returned with a bucket and eating utensils believed to have been made of scrap metal from his father’s plane. Joe’s remains have not been found.

Maureen Alice Hoey was born Sept. 23, 1940, in Boston, the daughter of Irish immigrants. She and Joe met on a blind date and were married in 1965.

She said their last hours together before he left for Vietnam in 1967 always remained clear in her mind, “as if it happened yesterday.”

“Tears came to my eyes . . . and Joe kept saying, ‘Please don’t cry. I’ll be fine. I promise you I’ll come back,’ ” she once recalled in an interview with the Herald. “The next day I took him to the airport and as he kissed me goodbye he told me again, ‘I promise, I’ll be back.’ Then he walked away.”

For years, Mrs. Dunn kept her husband’s name as their listing in the phone book. She lived in Randolph, Mass., and supported herself and her son as a hairdresser and a floral and interior designer. Having discovered her talent for political activism, she became the first woman elected to the town’s board of selectmen.

Mrs. Dunn confessed that activism did not come naturally to her. “They made me do this,’’ she told the Patriot Ledger of Quincy, Mass., referring to government officials and what she considered their insufficient cooperation. “They made me become a person I’m not fond of sometimes. But I had to do this to get this done.”

Survivors include her son, Joseph P. Dunn II of Hyannis, Mass.; two brothers; a sister; and a grandson, who, like her son, is named for her husband. The Dunns’ story is recounted in the book “The Search for Canasta 404: Love, Loss, and the POW/MIA Movement,” written by Mrs. Dunn and reporter Melissa B. Robinson.

Colleen Shine, of Arlington County, lost her father, an Air Force pilot, in the Vietnam War. Like Mrs. Dunn, she spent years trying to find him, or at least what became of him.

Because of her efforts, the remains of Lt. Col. Anthony Shine were discovered, and in 1996, he was buried at Arlington. She said Mrs. Dunn’s determination helped inspire her own.

“To look in the face of someone who had such a great love for her husband that never died,” Shine reflected. “Her loyalty to him was to the end.”