The Citizens Bank on Route 1 was closed at 11:15 a.m. June 17 after employees said they felt nauseous and lightheaded, police say.
Firefighters cleared the building and checked for carbon monoxide but did not detect it, said Cpl. Gary Fournier of the Delaware State Police.
Department of Natural Resources and Environmental Control agents also responded and detected no carbon monoxide in the building, he said.
Employees were sent home and the branch was scheduled to reopen Tuesday, he said.