Immigration agents have served a criminal search warrant on a Bellingham plant where 28 workers were arrested in February.

The search warrant was served Thursday morning on Yamato Engine Specialists.

U.S. Immigration and Customs Enforcement spokeswoman Lorie Dankers says it's part of an ongoing investigation, so she can't give more details.

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The workers arrested in February were suspected of being in the country illegally. Many were later released and given permission to work.

The raid on the engine manufacturing plant was the first mass arrest of immigrants since President Barack Obama took office. It appeared to contradict his policy that federal agents focus more on employers who hire undocumented workers than on the workers themselves.

Homeland Security Secretary Janet Napolitano has ordered a review of the raid.