Americans responded in overwhelming numbers Monday to the online release of detailed information from the 1940 census — the first time such a trove of historic census records has been available on the Internet.

Minutes after its launch, the 1940 census portal on the National Archives and Record Administration website was all but impenetrable. Officials apologized and promised the website would be accessible as soon as possible.

“In the first three hours, we had 22.5 million hits,” said Susan Cooper, spokeswoman for the National Archives. “We’re a victim of our own success.”

Cooper said the archives had anticipated significant interest in the release, which included nearly 4 million images of painstakingly handwritten census pages. Some people were able to access the site, but others endured long waits or were not able to get on at all, she said.

“It’s frustrating, and we share that frustration with the public,” Cooper said. “We’re working as fast as we can to fix the problem.”

Individual records from a decennial census become public every 10 years, as soon as the government’s legally mandated 72-year waiting period expires. But release of the 1940 documents had been awaited more eagerly than usual, partly because they will be searchable online. Genealogists, historians and family history buffs also say this particular census, the 16th conducted, sheds light on an especially interesting period of U.S. history.

Michael Snow, a Census Bureau historian, said the newly released information helps to fill in a portrait of the nation as it dug its way out of the Great Depression and stood at the brink of World War II. The records offer details on the period’s great westward migration, prompted by Depression-era job losses and the Dust Bowl conditions that hit the Great Plains about the same time.

A new question, Snow said — one of many on the 1940 census — asked people where they had lived five years earlier.

“People at the time talked about people they knew taking to the roads in great numbers,” the historian said. “Southern California, and the Central Valley in particular, were an epicenter of that migration. And this question was a way to capture that movement.”

Other information being released includes people’s names, ages, addresses, marital status and number of children. It also includes occupations, and for a sample of respondents, how much they earned.

The site cannot yet be searched using names. But plugging in an address or approximate location will lead to the appropriate “enumeration district” — the area a census taker covered.

Armed with the district’s number, researchers can locate and browse the scanned images of handwritten logs in order to find the names and addresses they are seeking.

About 21 million of the 132.2 million Americans counted in 1940 are still alive, officials said.

Among them is Jay Holladay, a retired engineer from La Crescenta who was then 3 years old and living with his family near Sacramento. Holladay said he looks forward to searching the records for information about his parents — especially his father, who died a number of years ago.

“My mom and dad had gotten married in 1935, and I know my dad had a little farming operation,” said Holladay, a longtime volunteer with the Southern California Genealogical Society. “That’s part of the fun of the chase with this. We moved to San Francisco a few years later, and he was doing something entirely different. I’ll be interested in what it says about how he described himself and where they were living.”

Holladay is the local society’s coordinator for its part in a national project to help the archives create a name-based index for the new website. Others involved in that effort include commercial family history sites such as Archives.com, as well as FamilySearch.org, the genealogy organization run by the Church of Jesus Christ of Latter-day Saints.

Some privacy advocates have sounded concerns about such a wide-scale, online release of information about living people. The American Civil Liberties Union, for example, has long opposed unrestricted use of census records.

“Just because information is old doesn’t mean it isn’t intensely personal to the people it’s about,” said Chris Calabrese, legislative counsel with the ACLU.

For example, he cited the case of someone born out of wedlock who might be embarrassed by census records that publicly confirm that fact.

The ACLU also is concerned about how technology could be used to combine the census’ rich trove of personal data with other information to build highly detailed, intrusive images of people’s lives, Calabrese said.

Snow said any risk of identity theft from the data release was alleviated by the fact that census-takers did not ask for birth dates or Social Security numbers. One 1940 census question did ask a sample of respondents if they had a Social Security number, he said, but not for the number itself.

The website is 1940census.archives.gov/.

rebecca.trounson@latimes.com