Where did they serve? What did they do and see? Spouses and children often learned not to ask. And by now, most no longer have the chance: Fewer than 3 percent of the 16 million American veterans of the war are still alive, and all are in their 90s or beyond.

But that has not kept their children and grandchildren from wanting to know their stories, especially as the 75th anniversaries of the D-Day invasion and the other triumphs of the war’s final year have neared. And a growing number of them are turning to experts to help glean what they can from cryptic, yellowed military records.

“We have people calling every day to try to find out about their fathers,” said Tanja Spitzer, a researcher at the National World War II Museum in New Orleans. “They regret that they didn’t do anything when their parents were alive. We get a lot of apologizing about it. For them, it is very emotional.”

Ms. Spitzer tells them it is not too late. Among the nation’s many staggering accomplishments in World War II were the billions of pages of personnel files that War Department and Navy clerks amassed to keep track of everyone in uniform. Most of those records still exist, stored in a climate-controlled facility in St. Louis by the National Archives and Records Administration.

The repository is immense, with enough boxes of files to stretch more than 545 miles. The boxes hold everything from the mundane, like payrolls and medical screening forms, to the heart-tugging: photos of young recruits, letters from worried mothers, medal citations. Researchers can use them to recreate the individual stories that many troops never told.