A tip from the Democratic mayor of Oakland, Calif., may have helped illegal immigrants convicted of crimes evade law enforcement, further endangering their communities.

After Mayor Libby Schaaf got word of an impending Immigration and Customs Enforcement raid in the Bay Area, she tweeted a warning to help potential targets of the operation prepare. “I know that Oakland is a city of law-abiding immigrants and families who deserve to live free from the constant threat of arrest and deportation,” Schaaf said at the time. “I believe it is my duty and moral obligation as mayor to give those families fair warning when that threat appears imminent.”