The girl's stepmother, 42-year-old Elisa Baker, was taken earlier this week to a home where she once lived in an effort to see what information she has and what she is willing to share.
The stepmother has been charged with obstruction of justice.
Police said she acknowledged to writing a bogus ransom note found at the scene of a fire in the family's back yard on the day Zahra Baker was reported missing.
In recent weeks, authorities have combed wooded areas and a landfill for the disabled child or evidence in her disappearance.
On Tuesday, investigators said they seized a mattress found by workers at a Caldwell County landfill following a search last week. Officials have said the mattress will be tested for DNA evidence.
The girl's father, Adam Baker, was arrested Monday on unrelated charges. He and his daughter are from Australia. He and his daughter moved to North Carolina in 2008 after he married Elisa Baker, whom he had met on the Internet.
Adam Baker, 33, is facing one count each of assault with a deadly weapon and failure to return rental property, two counts of communicating threats and five counts of writing worthless checks, authorities said.
The timing of his arrest raises questions because the charges against him were filed over the past few months. Hickory Police Chief Tom Adkins had said authorities delayed taking action because he was cooperating with police.
Deal said investigators haven't talked to Adam Baker since his arrest. He also said Adam Baker did not have an attorney.
Adam Baker was being held in a western North Carolina jail on $7,000 bond. Elisa Baker is at the same jail.
Police have not found the girl's body and haven't charged anyone with killing her.