On Feb. 26, 1931, a sunny Sunday in Los Angeles, hundreds gathered for an afternoon of relaxation in La Placita park in the heart of the city’s Mexican community.

Suddenly, a large group of plainclothes officers armed with guns and batons entered the park. Two officers were posted at each entrance to La Placita so that no one could leave. Dozens of flatbed trucks circled the park’s perimeter.

Officers rounded up all the people with brown skin, said Joseph Dunn, a former Democratic state senator from California, who researched this forgotten episode of U.S. history.

Panic swept through the crowd. About 400 park patrons were lined up and asked to show proof of legal entry and citizenship of the United States.

The Mexican immigrants and Mexican Americans who could not produce proper documentation were detained. Then, some were put on the trucks and sent to the city’s main railroad station, Dunn said. Once there, they were ordered onto previously chartered trains and taken deep into Mexico, according to Dunn.