A mysterious “Mexican suitcase” has been unpacked to reveal a treasure trove of classic photo-journalism taken in the 1930s by Robert Capa and two other pioneers.

The three cardboard boxes contain 126 rolls of 35mm film with about 4,300 images of the Spanish Civil War, most never seen before.

They were saved from wartime Europe and appeared in Mexico City half a century later among the effects of a former Mexican diplomat, before finding their way to New York.

The photos were taken by Capa, his lover and professional partner Gerda Taro, and David “Chim” Seymour, co-founder with Capa of the photo agency Magnum. They include images of the American writer Ernest Hemingway by Capa, the French author André Malraux by Taro and the Spanish poet Federico García Lorca by Seymour.

Times Archive, 1936: Civil war in Spain

Savage fighting, executions at San Roque, government gain in north

• Archive Topic: The Spanish Civil War

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“It is important historical material, as well as work by these three famous photographers,” Cynthia Young, curator at the International Centre of Photography in New York, said.

The rolls of film contain images by Capa of the Spanish Civil War, including destroyed buildings in Madrid, the Battle of Teruel, the Battle of Rio Segre, and the mobilisation for the defence of Barcelona in January 1939, as well as the mass exodus of people from Tarragona to Barcelona and the French border. The boxes also contain a series by Capa of the internment camps for Spanish refugees in Argelès-sur-Mer, Le Bacarès and Bram in March 1939.

“A few of those we saw published at the time. Now we have ten rolls of those images,” Ms Young said.

The film provides new insight into Capa’s working methods by showing the negatives on either side of published images. “In some cases, you can see his famous dictum: if your pictures are not good enough, you are not close enough,” Ms Young said. “In some, you can see him homing in on his subject.”

However, despite the initial hopes of experts, the discovery does not cast any light on the controversy over whether Capa staged his most famous shot, The Falling Soldier. The photograph shows a Spanish Republican militiaman falling backwards, apparently at the moment he is struck by a bullet near Córdoba on September 5, 1936. The negative of the iconic picture has never been found.

Almost all of the Capa images in the boxes date from 1937. They came into the possession of General Francisco Aguilar González, the Mexican Ambassador to the Vichy Government in 1941-42. The rolls of film were discovered among the general’s effects by the Mexican film-maker Benjamin Tarver, who inherited them from his aunt, a friend of the general. After prolonged negotiations, he sent them to the photography centre in New York.

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