New York ? It was a decade when tens of millions of people in the U.S. experienced mass unemployment and social upheaval as the nation clawed its way out of the Great Depression and rumblings of global war were heard from abroad.

Now, intimate details of 132 million people who lived through the 1930s will be disclosed as the U.S. government releases the 1940 census on April 2 to the public for the first time, after 72 years of privacy protection lapses.

Access to the records will be free and open to anyone on the Internet — but they will not be immediately name-searchable.

For genealogists and family historians, the 1940 census release is the most important disclosure of ancestral secrets in a decade and could shake the branches of many family trees. Scholars expect the records to help draw a more pointillistic portrait of a transformative decade in American life.

Researchers might be able to follow the movement of refugees from war-torn Europe in the latter half of the 1930s; sketch out in more detail where 100,000 Japanese-Americans interned during World War II were living before they were removed; and more fully trace the decades-long migration of blacks from the rural South to cities.

Henry Louis Gates Jr., a Harvard University professor and scholar of black history who has promoted the tracing of family ancestry through popular television shows, said the release of the records will be a “great contribution to American society.”

Gates, whose new PBS series “Finding Your Roots” begins March 25, said the “gold mine” of 1940 records would add important layers of detail to an existing collection of opened census records dating to 1790.

“It’s such a rare gift,” he said of the public’s access to census records, “especially for people who believe that establishing their family trees is important for understanding their relationship to American democracy, the history of our country, and to a larger sense of themselves.”

Margo Anderson, a census historian, said the release of the records could help answer questions about Japanese-Americans interned in camps after the outbreak of WWII.

“What we’ll be able to do now, which we really couldn’t do, is to take a look at what the Japanese-American community looked like on the eve of evacuation,” said Anderson, a professor of history and urban studies at the University of Wisconsin-Milwaukee.

More than 120,000 enumerators surveyed 132 million people for the Sixteenth Decennial Census — 21 million of whom are alive today in the U.S. and Puerto Rico, according to the U.S. Census Bureau.