Mexican police investigating the murder of two bus drivers last week are examining claims the killer was a woman seeking to avenge sexual abuse.
Two drivers from the same company in Ciudad Juarez were killed last week.
A woman who identified herself as "Diana the huntress" sent emails to newspapers and posted messages on social media saying she was the killer.
She said she murdered the drivers to avenge women passengers sexually abused by night-shift drivers.
"I and other women suffered... but we can't stay quiet," one of the emails said.
The Diario de Juarez newspaper reported that the killer did not rob the victims, nor did she use a weapon normally favoured by gang members.
On Wednesday, a woman with blonde wig or dyed hair approached the first driver, took out a pistol, shot him in the head and left the bus, the Associated Press news agency reported.
The killer told the second victim, "You guys think you're real bad, don't you?" before shooting him.
The message by the self-proclaimed murderer caught the attention of authorities because it contained certain details about the crimes that had yet to be publicly disclosed.
According to the alleged killer's message, many women became victims of sexual violence when they travelled by bus to the factories that employ many residents in Ciudad Juarez, across the border from El Paso in the US state of Texas.
"I am the instrument of vengeance for several women," the message said.
Police have not verified the authenticity of the email, or a Facebook page set up under a similar name on 31 August.
Undercover police will travel on board some buses and conduct weapons searches to prevent further killings.
"We have a police sketch of the suspect and we are looking for her,'' municipal police spokesman Adrian Sanchez said.
Ciudad Juarez has a history of sexual violence against women.
More than 100 women disappeared in Ciudad Juarez in the 1990s and early 2000s. Their bodies were often found weeks later, raped, strangled and dumped.
Several bus drivers were arrested in connection with those killings. One driver had his conviction overturned, and his co-defendant, another bus driver, died in prison before sentencing.