The murder of film director Leon Serment and his producer wife's subsequent death are believed to have been contract killings arranged by their son for the equivalent of about $5,200 per victim, Mexico City's top prosecutor said Friday.

Prosecutor Rodolfo Rios alleged at a news conference that the unnamed son and his girlfriend were the "intellectual authors" of the murders and said four suspects, including those who ordered and those who carried out the killings, were arrested Thursday.

The alleged plot was said to have been planned over the course of two months.

Serment directed the 2010 film "Tequila Effect," a thriller about the 1994 peso crisis. He was killed by two attackers while returning with his son to his home in the capital the night of Aug. 27, in what was believed at the time to have been a robbery gone bad. Investigators later found inconsistencies in the son's testimony.

Video evidence showed the attackers lying in wait and using their cellphones as others passed by before attacking Serment, suggesting they were targeting him specifically. Investigators also determined that just a half-hour after the killing, the attackers withdrew money from ATMs and made a purchase at a convenience store using the director's bank cards, indicating that they had access to his PINs.

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Serment's wife, Adriana Rosique, told police it was unusual for the son to visit his father and that the girlfriend had been sending cellphone messages while they were at his home.

"There was phone communication between the material authors with the son and his girlfriend," Rios said.

Rosique was found hanged in her home on Sept. 19. The son reportedly told police he had left the home that night to take his girlfriend to a hospital, and didn't learn of his mother's death until the following morning. But there was no record of their presence at the hospital; instead, video recordings showed them walking around streets in the area.

Rios said video cameras also captured the son leaving the home while using his cellphone, and two attackers entering with ease moments later.

"It is the son who leaves the door to the house open," the prosecutor said.

In a search of the home of one of the alleged attackers, investigators found a bank card in Rosique's name that had been used to make daily withdrawals since her death.

In an interview with Mexican journalist Joaquin Lopez-Doriga on Radio Formula, Rios said both victims had life insurance, the son apparently harbored a deep resentment of his parents and they were not accepting of his relationship with the girlfriend. He said that the day before Rosique's death there had been an argument at the home between her, the son and the girlfriend that came to blows.

The Mexican Academy of Cinematographic Arts and Sciences has identified Rosique as a producer. A website for Mexico City-based production company Taller de Luz lists her and Serment as its two contacts.

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