Investigators in Maryland are using genetic tests to determine if a Maryland woman was the mother of four dead fetuses found at her home .
FBI agents are now searching a dirt lot next to Christy Freeman's apartment.
Investigators first searched her home after they found a stillborn,
pre-term baby boy there last week.
They later found the remains of three more pre-term babies.
Freeman is charged with first and second-degree murder and manslaughter after the first body was found.
She is being held without bail.