(CNN) -- Investigators have narrowed down the date of the last time someone other than a family member saw a missing 10-year-old North Carolina girl to September 25, police said Saturday.
Zahra Baker was reported missing October 9, and Burke County Sheriff John McDevitt has said she is believed to be dead.
Her disappearance is being investigated as a homicide, and her stepmother has been arrested on charges of obstructing justice.
Police earlier had said that no one outside the girl's immediate family had reported seeing her for nearly a month. But officials recently said another witness had come forward to say Zahra was seen in mid- to late September.
Authorities have spent days combing an area about 20 miles north of Zahra's home in Hickory, North Carolina. On Thursday, as many as 70 searchers were backed by dogs and a backhoe that was used to clear piles of wood and mulch, McDevitt said.
The searches near Morganton started Tuesday as investigators focused on a lot where Zahra's father, Adam Baker, frequently worked hauling loads of wood and brush to be mulched. Wednesday, investigators came up empty after draining a pond next door.
Police said last week that the stepmother, Elisa Baker, admitted to writing a fake ransom note that was left at the family's home. She was arrested Sunday on an unrelated charge of passing bad checks, prosecutors said.
Elisa Baker remained jailed on those charges Thursday. Her bail has been set at a combined $70,000, an amount her lawyer called "excessive." Police said Adam Baker also faces bad-check charges, but is not being arrested while he assists police.
"At this point, he's doing what we ask him to do. If we need him to do something, he shows up," said Clyde Deal, Hickory's deputy police, on Thursday chief. Deal said other searches have been conducted, but the warrants have been sealed and no details are being released.
Zahra reportedly suffered from bone cancer and used a prosthetic leg and hearing aids. The hearing aids have been found, but not the prosthesis, Hickory Police Chief Tom Adkins has said.
Relatives and friends of the family have alleged that Elisa Baker physically abused Zahra and said she was reported to state social services officials. The North Carolina Department of Health and Human Services has declined comment on the case.
Hickory is about 60 miles northwest of Charlotte.
HLN's Natisha Lance contributed to this report.