As a teenager in World War II, Aiko Herzig Yoshinaga was removed from her high school and imprisoned in an internment camp for Japanese Americans. Her research later led to reparations for the camps’ prisoners and an apology from the president. (Gina Ferazzi/Los Angeles Times)

Early in 1942, two months before Aiko Herzig Yoshinaga was scheduled to graduate from high school in Los Angeles, her principal assembled the school’s 15 Japanese American seniors for a meeting.

Mrs. Herzig Yoshinaga was an honors student, with plans to study music and art in college, but President Franklin D. Roosevelt had just signed Executive Order 9066. Nearly 120,000 Japanese Americans were being forced into internment camps.

“You all don’t deserve to get your high school diplomas because your people bombed Pearl Harbor,” she recalled her principal saying.

By graduation day, Mrs. Herzig Yoshinaga — the American-born daughter of Japanese immigrants — was sleeping on a hay-filled sack at the Manzanar War Relocation Center, a dust-blown “prison camp,” as she later called it, in the shadow of the Sierra Nevada range.

Mrs. Herzig Yoshinaga would spend decades trying to forget the war years, until the anti-Vietnam War movement gave her a new political sensibility and drew her back into the past. Settling in the Washington area in 1978, she began visiting the National Archives, searching for information on internment.

She was soon spending six days a week in the archives, scouring thousands of poorly indexed documents. With her husband, she copied so many documents that they filled a bathtub in their home, enough copies that their master bedroom was lined with file cabinets and converted into an office.

Mrs. Herzig Yoshinaga was honored at Los Angeles City Hall in 2011. Behind her are son-in-law Warren Furutani; the City Council president, Eric Garcetti; and Mayor Antonio Villaraigosa. (Mario Gershom Reyes)

And then, in the early 1980s, Mrs. Herzig Yoshinaga picked up a red bound volume sitting on the corner of an archivist’s desk. As she later told the Los Angeles Times, the book contained the original draft of a 1943 government report on internment, apparently the last such copy in existence. “I began thumbing through the report, and then, when I came upon an important section, I nearly hit the ceiling.”

Mrs. Herzig Yoshinaga, who was 93 when she died July 18 at a hospital in Torrance, Calif., had uncovered evidence suggesting America’s World War II internment policy had racist motives and was not a result of “military necessity,” as Pentagon officials claimed.

Her findings helped persuade Congress to pass the 1988 Civil Liberties Act, which granted $20,000 in reparations to each survivor of the camps and a formal apology from President Ronald Reagan.

They also proved instrumental in a 1983 legal effort to overturn the criminal conviction of Fred Korematsu — a welder who had defied orders to report to an internment center, and who unsuccessfully challenged Roose­velt’s executive order before the Supreme Court.

Similar convictions for two other Japanese Americans, Gordon Hirabayashi and Minoru Yasui, were subsequently cleared with the help of Mrs. Herzig Yoshinaga’s “pivotal” research, said Dale Minami, a San Francisco-based lawyer who led the legal team in all three cases.

“What she did was expose misconduct in the government in this dark light — the alteration of an originally racist justification to a more benign one, to make it more palatable to the Supreme Court,” Minami said in a phone interview.

A former secretary and stenographer, Mrs. Herzig Yoshinaga said her efforts were inspired by Michi Nishiura Weglyn, author of “Years of Infamy: The Untold Story of America’s Concentration Camps” (1976), who urged her to visit the National Archives if she wanted to learn more about wartime internment.

By 1980, when a congressional commission was established to study the motivations and effects of Executive Order 9066, Mrs. Herzig Yoshinaga had gathered about 8,000 documents. The Commission on Wartime Relocation and Internment of Civilians hired her as a researcher.

Three years later, in a 467-page report written by lawyer Angus C. Macbeth, the commission concluded that internment was prompted by “race prejudice, war hysteria and a failure of political leadership.”

While officials in the Roosevelt administration insisted internment was essential to the war effort, the commission reported that “not a single documented act of espionage, sabotage or fifth column activity was committed by an American citizen of Japanese ancestry or by a resident Japanese alien on the West Coast.”

The report singled out Mrs. Herzig Yoshinaga for praise, declaring that she “in large part found and organized and remembered the vast array of primary documents from which the report was written.”

No document proved more significant than the one in the little red book. Written by Lt. Gen. John L. DeWitt, it said that internments were necessary not because there was “insufficient time” to distinguish loyal and disloyal Japanese Americans, but because “an exact separation of the ‘sheep from the goats’ was unfeasible.” The nature of the Japanese “race,” in other words, made internment essential.

Mrs. Herzig Yoshinaga and her research partner, lawyer Peter H. Irons, also uncovered a certificate “verifying that all copies of that report were burned,” Minami said. “So there was destruction of evidence as well. Inexplicably, one copy of the report remained.” The published version, featuring altered language, was used as evidence to uphold the internment policy in court.

In 2011, the acting U.S. solicitor general declared that the government had suppressed evidence when the Korematsu case was heard before the Supreme Court in 1944. Then, a month ago, Chief Justice John G. Roberts Jr. formally overruled Korematsu, declaring that “the forcible relocation of U.S. citizens to concentration camps, solely and explicitly on the basis of race, is objectively unlawful.”

The decision was perhaps the final step in a process of national atonement spurred by Mrs. Herzig Yoshinaga’s research, the success of which shocked even her.

“I’m just a little old housewife. I’m not a professional archivist,” she told the Los Angeles Times in 1988. “But I guess I showed that one person can make a difference.”

The fifth of six children, Aiko Louise Yoshinaga was born in Sacramento on Aug. 5, 1924, and grew up in Los Angeles. Her parents, immigrants from the Japanese island of Kyushu, managed a hotel and ran a produce stand.

When internment began, Mrs. Herzig Yoshinaga’s parents and siblings were sent to the stables at Santa Anita racetrack in Arcadia, Calif., and then to camps in Arkansas. To avoid being separated from her boyfriend, Jacob Miyazaki, Mrs. Herzig Yoshinaga eloped. The couple were bused 250 miles to Manzanar, near Independence, Calif., where Mrs. Herzig Yoshinaga gave birth to a daughter, spent hours washing sand and dust from her newborn’s diapers and saw her father only on his deathbed, while he was ailing at a separate camp in Arkansas.

After the war Mrs. Herzig Yoshinaga divorced her husband and married an Army officer, Davis Abe. That marriage also ended in divorce, and Mrs. Herzig Yoshinaga joined her mother and several siblings in New York City, where she took classes to receive her general equivalency diploma.

Amid protests over the Vietnam War, she joined a progressive political group, Asian Americans for Action. (She was later arrested while protesting apartheid outside the South African Embassy alongside former president Jimmy Carter’s 17-year-old daughter, Amy.)

In 1978, she married Jack Herzig, a lawyer who had served as an Army paratrooper in World War II, and moved to Falls Church, Va. They worked as researchers for the National Council for Japanese American Redress, an early reparations effort led by William Hohri, before Mrs. Herzig Yoshinaga joined the congressional commission.

She had recently been hospitalized after a fall at her home in Gardena, Calif., said her son-in-law Warren Furutani, a former California state lawmaker who confirmed her death. Her husband died in 2005.

Survivors include a daughter from her first marriage, Gerrie Lani Miyazaki of Los Angeles; two children from her second marriage, Lisa Furutani of Gardena, Calif., and David Abe of Huntington Beach, Calif.; a brother; six grandchildren; and two great-grandchildren.

Mrs. Herzig Yoshinaga, who came to display a piece of barbed-wire fencing from Manzanar in her home as a reminder of the war years, helped former inmates gain redress after reparations began being paid in 1990. She later edited a collection of testimony from imprisoned Japanese Americans, “Speaking Out for Personal Justice” (2011), and was featured in a 2016 documentary, “Rebel With a Cause.”

Among her other projects was a dictionary of internment terms, published in 2009, which called for words like “internment camp” to be replaced with “gulag” or “concentration camp,” in an effort to accurately describe what happened during the war.

“For 40-plus years I’ve used the word ‘evacuation,’ because I was brainwashed to,” she told The Washington Post in 1988. “I’m trying very hard to use words like ‘banishment,’ ‘exile,’ ‘forced removal.’ In the camps, they called us ‘resident colonists.’ ”

“We must learn,” she added, “to tell it as it happened.”