Passage: Another theory was brought forward in February 2002 by a woman identified only as "Martha." Police considered "Martha"'s story to be plausible but were troubled by her testimony, as she had a history of mental illness. "M" claimed that her abusive mother had "purchased" the unknown boy (whose name was Jonathan) from his birth parents in the summer of 1954. Subsequently, the boy was subjected to extreme physical and sexual abuse for two and a half years. One evening at dinner, the boy vomited up his meal of baked beans and was given a severe beating, with his head slammed against the floor until he was semiconscious. He was given a bath, during which he died. These details matched information known only to the police, as the coroner had found that the boy's stomach contained the remains of baked beans and that his fingers were water-wrinkled."M"'s mother cut the boy's distinctive long hair (accounting for the unprofessional haircut which police noted in their initial investigation) in an effort to conceal his identity. "M"'s mother  forced "M" to assist her in dumping the boy's body in the Fox Chase area. "M" said that as they were preparing to remove the boy's body from the trunk of a car, a passing male motorist pulled alongside to inquire whether they needed help. "M" was ordered to stand in front of the car's license plate to shield it from view while the mother convinced the would-be Good Samaritan that there was no problem. The man eventually drove off. This story corroborated confidential testimony given by a male witness in 1957, who said that the body had been placed in a box previously discarded at the scene.In spite of the outward plausibility of "M"'s confession, police were unable to verify her story. Neighbors who had access to "M"'s house during the stated time period denied that there had been a young boy living there and dismissed "M"'s claims as "ridiculous."

Question: Why were police bothered by Martha's story?
she had a history of mental illness