A woman has been murdered in the Mexican border city of Nuevo Laredo, reportedly for posting information about a drug cartel on a social networking site.

It is believed this could be the third murder in a month that has been motivated by what people have said about drug cartels on the internet.

Marisol Macias Castaneda was found decapitated this weekend, alongside a note that referred to the nickname she apparently used on the site, 'Laredo Girl'.

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Nuevo Laredo en Vivo, or 'Nuevo Laredo Live', is a local social networking site that features tip hotlines for the Mexican army, navy and police. It also has a section for reporting locations of drug gang lookouts and drug sales points. It is believed this could be what has angered the cartel.

The note found next to the body read: "Nuevo Laredo en Vivo and social networking sites, I'm The Laredo Girl, and I'm here because of my reports, and yours. For those who don't want to believe, this happened to me because of my actions, for believing in the army and the navy. Thank you for your attention, respectfully, Laredo Girl … ZZZZ."

The letter Z refers to the notoriously violent Zetas drug cartel.

Fellow posters soon began discussing the murder online and described Castaneda as a frequent poster to the site.

Earlier this month a man and a woman were found hanging from an overpass in the same town, accompanied by a similar note that warned "this is what will happen" to other internet users.

It is unclear how the cartels discovered the true identities behind posters' online nicknames.