Authorities are seeking a woman accused of killing two bus drivers in northern Mexico amid claims the murders were committed by a vigilante avenging rapes, officials say.

Local media have received an anonymous email signed by "Diana, bus driver hunter" claiming to act as "an instrument of vengeance" for sexual abuse committed by drivers in the city of Ciudad Juarez.

Chihuahua state prosecutor's office spokesman Arturo Sandoval said 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 last week.

Allegations 12 women raped by bus driver

Mr Sandoval said prosecutors believe they were either crimes of passion or motivated by vengeance.

The drivers were working on a route used by women who work in assembly plants known as "maquiladoras," and who regularly suffer sexual abuse as they travel to work for night shifts.

Authorities are investigating 12 cases of female passengers allegedly being sexually assaulted by a driver.

Investigators are looking into whether the killer is among the women.

Officials are also investigating any possible links with an arson attack on a bus at dawn on Tuesday.

The vehicle was set ablaze after petrol was poured on it, fire chief Ramon Lucero said.

The anonymous email message from "Diana" stated: "My colleagues and I have suffered in silence, but they can no longer keep us quiet."

"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.

"They think that we are weak because we are women.

"I am an instrument of vengeance."

City has history of sexual violence against women

Authorities have drawn up a profile of the suspected killer and launched an operation to find her with undercover agents travelling on buses.

Witnesses describe her as a woman in her 50s, 165 centimetres tall, with a dark complexion.

Ciudad Juarez, which is near the border with Texas, gained notoriety in the 90s 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).

In several cases, the victims disappeared after they left the plants at dawn.

AFP