NEW YORK (AP) — 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 "goldmine" 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.

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"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.

The survey contained 34 questions directed at all households, plus 16 supplemental questions asked of 5 percent of the population. New questions reflected the government's intent on documenting the turbulent decade, by generating data on homelessness, migration, widespread unemployment, irregular salaries and fertility decline.

Some of the most contentious questions focused on personal income and were deemed so sensitive they were placed at the end of the survey. Less than 300,000 people opted to have their income responses sealed.

In part because of the need to overcome a growing reluctance by the American public to answer questionnaires and fears about some new questions, the bureau launched its biggest outreach and promotional campaign up to that time, according to records obtained at the Franklin D. Roosevelt Presidential Library and Museum in Hyde Park, N.Y.

It opened its first Division of Public Affairs to blanket the country with its message, reaching out to over 10,000 publications and recruiting public officials, clergy and business owners to promote it.

Movie studios were enlisted to encourage their film stars to participate, including Cesar Romero, who later played the Joker in the Batman television series. A photograph of President Franklin D. Roosevelt taking the census also was used for the campaign.

The bureau also hired the managing editor of "Opportunity: A Journal of Negro Life" to galvanize support in the black community. However, studies in the 1940s revealed undercounts, including 13 percent of draft-age black men.

In a first for the National Archives and Records Administration, the nation's recordkeeper plans to post the entire census on the Internet — its biggest digitization effort to date.

That might be unsurprising given that increasingly popular online ancestry services make vast amounts of genealogical data available. But for previous decennial census releases, researchers had to trek to NARA branches to crank through microfilm machines.

Still, finding a name in the 3.8 million digitized images won't be as easy as a Google search: It could be at least six months after the release before a nationwide name index is created.

In the meantime, researchers will need an address to determine a census enumeration district — a way to carve up the map for surveying — to identify where someone lived and then browse the records.

Some experts said enthusiasm for the release could be dampened by the lack of a name index, especially for novices.

"It may very well frustrate the newcomers," said Thomas Macentee, an industry analyst helping recruit volunteers for a name indexing effort sponsored in part by the Mormon-run FamilySearch.org. "It's like showing up on Black Friday. If you really want that TV set, if you really want that census record, you are going to be ready to go and you are going to keep at it no matter what."

Publicly-traded Ancestry.com, which has over 1.7 million customers, is also working to make the census records searchable by indexing almost all fields and providing proprietary tools to mine the data.

Josh Hanna, a senior adviser for the company, said the 1940 census will be the biggest database of its kind. "It'll be the deepest level of indexing we've ever done," he said. Access to the index and tools will be available for free through the end of 2013.

Other individuals and organizations across the country are also working to ease the use of the records, including the New York Public Library, which is digitizing the full set of New York City's 1940 telephone books to help people locate addresses.

Genealogy societies and libraries also have been holding packed workshops to educate their members.

In January, about three dozen people gathered in Manhattan for a meeting of the MetroNY Genealogy & Computers Special Interest Group to discuss the census. They included Michelle Novak, who has spent six years searching for information about her paternal grandfather, but has no street address to help locate him.

Novak, 43, said family members recalled him as a heavy drinker who worked long hours for the Pennsylvania Railroad and abandoned his family in the early 1930s.

But the few records she has been able to find include a signature in a railroad pension book. She believes the 1940 census might hold additional answers.

"If I can find one record, anything, it may help," she said in an email after the meeting. "Even if I find him in jail or deceased, at least I will have an answer."

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Online:

http://1940census.archives.gov

http://www.ancestry.com/1940

http://www.the1940census.com

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Follow Cristian Salazar at http://twitter.com/crsalazarAP and Randy Herschaft at http://twitter.com/HerschaftAP.