Two bus drivers were killed last week, and during the weekend someone sent an email to several news organizations not just accepting responsibility but heralding a mission to exact vengeance, the Los Angeles Times reported.
"You think because we are women we are weak, and maybe we are," the message said. "But only to a certain point. ... We can no longer remain quiet over these acts that fill us with rage.
"And so, I am an instrument who will take vengeance."
It was signed: "Diana, Huntress of Bus Drivers."
Authorities have yet to determine whether the message was sent by the actual killer or killers, or whether "Diana" is a woman or man.
The newspaper said the message addressed the issue of women who work nights in the city's manufacturing plants only to be attacked by the drivers of the buses the women ride home. The newspaper notes hundreds of women have been killed or disappeared in Ciudad Juarez in the past two decades.
The Times reported witnesses told the city's El Diario newspaper in both slayings a woman dressed in black with dyed blond hair, or possibly wearing a wig, stopped the bus and upon boarding drew a pistol and shot the driver twice in the head.
"We have undercover police, dressed in civilian clothes, riding the buses in hopes of preventing another such murder and also to try to find this woman," Arturo Sandoval, spokesman for the prosecutor's office in the state of Chihuahua, told the Times in a phone interview.
One unidentified bus driver told El Diario drivers "go around terrified, but you have to work," the Times said.
Imelda Marrufo, coordinator of a network of women's organizations, told the Times by phone she didn't know if Diana the Huntress is real.
"But if it is confirmed, remember, we are talking about a victim, someone who was raped and has probably lived with such a lack of justice that she has no hope that whoever did that to her will ever pay for the crime," Marrufo said. "Like so many women in Ciudad Juarez."