Searchers gathered in Bolingbrook, where Stacy Peterson was last seen in late October.
"Right now we're looking for a body," said Cassandra Cales, Peterson's younger sister. "We're looking for grave sites ... anything."
Investigators are treating the case as a possible homicide and have named Peterson's husband, former Bolingbrook police officer Drew Peterson, as a suspect. A grand jury is looking into both her disappearance and the death of his ex-wife, Kathleen Savio.
Drew Peterson, who hasn't been charged and denies any wrongdoing in both cases, has said Stacy left him for another man. A message left for Drew Peterson's attorney Saturday evening was not returned.
The search was set to continue on Sunday and then every following weekend, as long as enough funds are available to cover fuel and other expenses, organizers said.
"As long as it takes -- that's our new motto," said Roy Taylor, a search organizer. "We will find Stacy."