Employees at a Lenoir landfill Tuesday discovered a mattress they say may have belonged to Zahra Baker, police said, possibly providing a breakthrough in the nearly three-week search for the missing 10-year-old girl.

Workers were grading the Foothills Environmental Landfill when they uncovered a mattress, which investigators said matches the description of the one used by Zahra.

Investigators last week called the mattress a key evidence that could help them establish an accurate time line for Zahra's disappearance - and possibly yield DNA evidence.

Police said they gave the mattress to the State Bureau of Investigation for testing.

Authorities are calling the case a homicide.

The girl, who used a prosthetic leg due to bone cancer, has not been seen since Sept. 25, they said.

Police jailed Zahra's stepmother, Elisa Baker, 42, saying she admitted writing a fake ransom note. The girl's father, Adam Baker, 33, was jailed Monday on charges unrelated to Zahra's disappearance.

Police confirmed that Elisa Baker is assisting them in the investigation.

The Associated Press contributed to this report.