Local media received an anonymous message yesterday signed by "Diana, bus driver hunter," claiming the killings were payback for the sexual abuse committed by drivers in Ciudad Juarez, a border city with a dark record of violence against women.
Arturo Sandoval, a spokesman for the Chihuahua state prosecutor's office, told AFP that the email, sent over the weekend, "has been included in the investigation."
Witnesses said a woman wearing a blonde wig shot the drivers in the head after stopping the buses. Sandoval said prosecutors believe they were either crimes of passion or motivated by vengeance.
"My colleagues and I have suffered in silence, but they can no longer keep us quiet," the anonymous message said.
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"We were victims of sexual violence by drivers who worked during the night shift at the (plants) in Juarez. While many people know about our suffering, nobody defends us or does anything to protect us," it said.
"They think that we are weak because we are women," the message said, warning that there would be more deaths.
Authorities have drawn up a profile of the suspected killer and launched an operation to find her with undercover agents in buses.
Witnesses describe her as a woman in her 50s, 1.65 meters tall, with a dark complexion.
Ciudad Juarez, which lies at the border with Texas, gained notoriety in the 1990s when the bodies of hundreds of women began to appear in the desert bearing signs of extreme sexual violence.
Many of the victims were women who had come from other parts of the country to work in one of the city's assembly plants following a manufacturing boom generated by the North American Free Trade Agreement (NAFTA).