Investigators in North Carolina are testing DNA found on a mattress for any evidence in the case of a missing North Carolina girl presumed dead.

Authorities seized a mattress found by workers at a landfill in Caldwell County, N.C., on Tuesday, looking for any clues in the disappearance of 10-year-old Zahra Clare Baker.

"This mattress possessed DNA evidence that will be tested in the future to confirm if it is involved with this investigation," the Hickory Police Department said in a statement Tuesday.

Zahra, a bone cancer survivor who uses hearing aids and has a prosthetic leg, was last seen in public on Sept. 25. She was reported missing by her father on Oct. 9.  

Police believe the girl is dead, and have charged her stepmother, Elisa Baker, with felony obstruction of justice. Authorities say Baker confessed to writing a fake ransom note found at the scene of a fire in the family's backyard on the day Zahra was reported missing.

On Monday, authorities also arrested the girl's father, Adam Baker, on several charges unrelated to the case -- including assault with a deadly weapon,  communicating threats and writing worthless checks. 

The Hickory Police Department told FoxNews.com that Elisa Baker, 42, is now cooperating with authorities. 

The woman has been talking with investigators after her lawyer approached police offering her help, said Maj. Clyde Deal, deputy police chief in Hickory where Zahra lived. 

Adam Baker and his daughter are from Australia. He and Zahra moved to North Carolina in 2008 after he married Elisa Baker, whom he had met on the Internet.

FoxNews.com's Cristina Corbin and The Associated Press contributed to this report