Melissa Ann Best, 34, hasn't been seen since she dropped one of her children off at a friend's last Friday. Her husband and friends have put up a Facebook page hoping it will lead to information on her whereabouts. Best is from Round Lake Park.
An FBI spokesman says the request for federal help may mean the police don't think Best left on her own.
"If they have evidence that they think it's a runaway, a minor or an adult and they left intentionally, voluntarily on their own, generally they're not going to be asking for our help," said Ross Rice, FBI.
Best is unusually hard to track because she doesn't have a cell phone or credit cards.