SAN FRANCISCO (AP) — A federal immigration official says about 800 people living illegally in Northern California avoided being arrested because Oakland Mayor Libby Schaaf warned that immigration raids were upcoming.

U.S. Immigration and Customs Enforcement chief Thomas Homan told "Fox and Friends" Wednesday that Schaaf's action was "no better than a gang lookout yelling 'police' when a police cruiser comes in the neighborhood."

Homan says the Justice Department is looking into whether Schaaf obstructed justice.

The mayor warned residents Saturday night of large-scale raids by immigration agents in the San Francisco Bay Area.

The agency on Tuesday said that agents arrested more than 150 people in California after the mayor's warning in a sweep that covered cities from Sacramento to Stockton in California's Central Valley agricultural heartland.