A Separation

Simin wants to leave the country with her husband, Nader, and daughter, Termeh, as the former does not want Termeh to grow up under the prevailing conditions. This desire is not shared by Nader, who is concerned for his father, who lives with the family and suffers from Alzheimer's disease. When Nader determines to stay in Iran, Simin files for divorce.
The family court judges the couple's problems insufficient to warrant divorce and rejects Simin's application. She leaves her husband and daughter and moves back in with her parents. On the recommendation of Simin, Nader hires Razieh, a young, deeply religious woman from a poor suburb, to take care of his father while he is at work. Razieh has applied for the job without consulting her hot-tempered husband, Hodjat, whose approval, according to tradition, would have been required. Her family is financially dependent on the job, and she takes her daughter to the house with her.
Razieh soon becomes overwhelmed by taking care of Nader's father, which is physically and emotionally demanding. She finds the work very heavy, especially as she is pregnant.
One day, Nader and Termeh return to discover her grandfather lying unconscious on the floor in his bedroom, with one of his arms tied to the bed. Razieh is nowhere to be found. When she returns, Nader accuses her of neglecting his father and of having stolen money from his room (unbeknownst to Nader, Simin used the money to pay movers). Outraged, Nader shoves Razieh out of the apartment. She falls in the stairwell outside. Hodjat's sister later calls Simin to inform her that Razieh is in the hospital because she has suffered a miscarriage.
A prosecution is assigned to determine the cause of the miscarriage and Nader's potential responsibility for it. If it is proved that Nader knew of Razieh's pregnancy and caused the miscarriage, he could be sentenced for murder. Nader accuses Razieh of neglecting his father. The hot-headed and aggressive Hodjat physically confronts Nader on several occasions, and threatens him, his family, and Termeh's teacher, who testifies on Nader's behalf. When Hodjat is detained for another outburst, Razieh reveals that he has mental disorders, and that he is under therapy, leading to his release. Nader learns that the reason Razieh was absent that day was because she had gone to visit a physician, which Razieh had avoided revealing earlier. This news, combined with Hodjat's explosive temper, causes Nader to wonder if Hodjat is physically abusive to Razieh and had caused her miscarriage.
Termeh protects Naderâs position with a false statement and Simin, fearing for Termehâs safety, attempts to arrange a financial deal with Razieh and Hodjat, to pay blood money for the loss of their child. Nader is initially outraged by Simin's suggestion, as he feels that it would be an admission of guilt. But he also must admit that he lied about his knowledge of Razieh's pregnancy. However, Razieh has serious doubts as to whether Nader's actions caused the miscarriage; considering she had earlier been hit by a car while retrieving Nader's father when he had wandered out of the apartment and had first experienced symptoms of the miscarriage that night. After another full-blown argument, Simin forces Termeh to leave with her. Nader agrees the payment if Razieh swears to Quran that he is the cause of her miscarriage. Since she has doubts, she refuses, though Hodjat tries to force her.
Later, at the family court, Nader and Simin have filed for a divorce once again. Their mourning clothes implies his fatherâs death. The judge makes their separation permanent, and asks Termeh about which parent she chooses to live with. She states that she has made a decision, but asks that the judge tell her parents to wait outside before she tells him. Her choice is not revealed.

Question: Why is the prosecution assigned?
Answer: To determine the cause of the miscarriage and Nadar's potential responsibility for it.