CAIRO, Ill. — Residents hear mice rustling in the walls at night. Some occupants leave ovens on in the winter, their doors perched open, because furnaces fail. Ceilings droop from water damage, mold creeps across walls, and roaches scramble out of refrigerators.

So when federal authorities finally deemed two public housing developments here in the southernmost tip of Illinois unacceptable and uninhabitable, it felt like vindication of what residents had been saying for ages.

But then came the solution: an order that everyone must vacate.

The authorities announced last month that the cost of fixing the developments was out of reach, and that replacing them altogether would cost almost 10 times as much. For hundreds of residents, the decision may mean not only leaving these crumbling buildings, but also moving from Cairo altogether.

“For sure they needed to fix this place up a long time ago,” said Nena Ellis, 38, a mother of three who lives in the McBride development, which, along with a second complex, Elmwood, is now set for demolition. “But there’s really nowhere else for us to go around here — even with a housing voucher, there just aren’t other places.”