Within hours of the release of 1940 U.S. Census data Monday morning, Kathy Amoroso had looked up her father.

He was 9 years old and living with his mother, father, brother and sister at 101 Elm St. in Lewiston when the census taker knocked on their door 72 years ago.

She discovered that her grandfather, a salesman, earned $2,300 in 1939.

“It brings history to life,” said Amoroso, a genealogy buff who is director of digital projects for the Maine Historical Society.

Amoroso was one of many people who trolled through data on millions of mid-20th century Americans that became publicly available for the first time Monday.

Every 10 years, the U.S. National Archives releases previously confidential census data from 72 years earlier — the average human life span. This is the first time the information has been released in digital form, so that actual census forms can be viewed online.

The records are the original written forms filled in by the census takers. They detail the number of people living in each household, their relationship to each other, education level, place of birth, employment status, occupation, marital status, even the value of their home or how much the residents paid in rent.

“This is a whole new sandbox to play in,” said Peggy O’Kane, head of reference at the Maine State Library in Augusta.

Genealogists and other historical researchers eagerly awaited the release. Interest in family history has surged in recent years, fueled by online genealogical research sites and television shows such as NBC’s “Who Do You Think You Are?” which follows celebrities as they trace their family trees.

The newly public information is expected to shine new light on the lives of the so-called Greatest Generation, who came of age during the Great Depression and went on to endure, and win, World War II.

For now, it’s possible to search geographically for only some parts of Maine and other states. In the next few months, as masses of volunteers help index the data, it will be possible to search more widely, and by name.

Paul Doucette of Gorham, president of the Greater Portland Chapter of the Maine Genealogical Society, will be among those helping out.

“I will be sitting at a computer for an hour or two a day, typing in information as it comes in from the census,” he said.

Doucette was so excited about the release that he spent his lunch hour Monday using the 1940s census database, researching his wife’s family in Bethel. He said the process was painstaking and slow, but it will get much faster and easier with indexing.

The government website was slow Monday because so many people were using it. O’Kane said that ancestry.com, a subscription website that’s available free at every Maine public library, is much faster, at least for now.

She looked up a census block on Forest Avenue in Portland, formerly the Maine Home for Friendless Boys, and discovered that 23 people were living at the home in 1940, from a variety of birthplaces in New England.

O’Kane also discovered the names and occupations of the people who, in 1940, lived in the home that is hers today.

O’Kane said the census information can reveal surprises.

“You could find out that your father enlisted in the Army when he was only 15 and has been lying about his age ever since,” she said.

Amoroso was so intrigued about the 1940 census data that she flew to New York for a daylong workshop in advance of the data dump.

“I know, I am a geek; but there were 500 other people there,” Amoroso said.

She said that in her search of the 1940 census, she is looking for new details about the Boston branch of her family and trying to solve the mysterious disappearance and reappearance of a female relative.

“The census is a jumping-off point to so many other records,” she said.

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