Surveillance photos show a person dressed in black, wearing a bandanna, placing the fliers around 1:15 a.m.
The text included a well-known white-supremacist slogan: "We must secure a future for our people and a future for WHITE children."
The fliers were placed just hours after the Long Beach City Council voted to approve turning the city's convention center into a shelter for unaccompanied migrant children crossing the border from Mexico.
At the request of the U.S. Department of Health and Human Services, Office of Refugee Resettlement, local governments throughout the nation are being asked to provide temporary facilities for unaccompanied migrant children while they are reconnected with family members or sponsors. Federal officials say existing Border Patrol facilities are overcrowded.
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In response to that request, Long Beach city officials explored potential sites and determined the convention center could provide housing, food, recreational and other services to children in need.
It could temporarily accommodate up to 1,000 children for a period of 90 to 120 days.
"These kids deserve our care, love, and support,'' Long Beach Mayor Robert Garcia tweeted after the 9-0 vote. We will work to ensure that every child is quickly reunited with family.''
City News Service contributed to this report.