NUEVO LAREDO, Mexico (Reuters) - Maria Macias used the Internet to denounce the brutality of local drugs lords in this Mexican border city until they silenced her and dumped her headless body on a busy street.

Macias, 39, had been an active contributor to a chat room on a community website, "Nuevo Laredo en Vivo" (Nuevo Laredo Live), where she posted information about crimes and urged fellow citizens to do the same.

The site lists official hotline numbers to the police and army to report crimes in the city just across the border from Laredo, Texas.

"Keep speaking out," she wrote last Thursday under the handle "NenaDLaredo" (or Laredo Girl), according to messages still visible on the site, urging readers to catch the "ratzzz" -- an apparent reference to the brutal Zetas drug gang.

Her mutilated body was found on Saturday morning at a busy intersection, next to computer keyboards and a sign saying: "OK Nuevo Laredo en Vivo and social networks, I am Laredo Girl and I am here because of my reports and yours."

It was signed ZZZZ.

The website now has a black ribbon next to her web nickname saying "1972-2011, Rest in Peace."

But the newspaper where she worked in an administrative post, according to other local journalists, wrote only a small story about the body without mentioning her name, a sign of the self-censorship imposed by many local newspapers and TV and radio stations that have been threatened and in some cases infiltrated by drug gangs.

Macias' killing, and that of two other people believed to be active users of social media in Nuevo Laredo, shows cartels here are now turning their focus to the websites and social media networks which are flourishing as an alternative source of information about drug violence in Mexico.

Mexicans are avid fans of Twitter, and Internet use has doubled in the last six years.

Online offerings include official anonymous tip sites, specialist blogs which carry explicit photos of cartel murder victims and neighborhood watch-style sites run by local citizens or newspapers.

But the drug gangs are also cyber-savvy. They have used YouTube to post videos showing how they torture and murder rivals, often forcing confessions before an on-camera execution.

And after silencing local news media in many areas, they are now going after their critics in social media.

"The narcos have people who are experts in communications," said a journalist from Tamaulipas state, home to Nuevo Laredo, who asked to remain anonymous. "They are monitoring Internet sites, blogs, phone calls and the famous social networks on a daily basis."

The newspaper where Macias worked refused to comment on her death. Authorities gave her job title there as director of information.

Tamaulipas is one of the states hardest-hit by the violence, which has killed about 42,000 people since President Felipe Calderon launched a crackdown on cartels in late 2006.

The Zetas gang is fighting a vicious battle with its former employer, the Gulf Cartel, in the state to control the local drugs market as well as lucrative smuggling routes to the United States.

ALTERNATIVE SOURCES

At least 42 Mexican journalists have been killed in the past five years, according to the Committee to Protect Journalists, which has called for an inquiry into Macias' death. Mexico's human rights commission puts the number of dead journalists even higher.

The killings have effectively gagged the media in states overrun with drug violence -- like Tamaulipas.

"Every day, Mexicans' right to know is violated as there are more areas of the country where information is silenced," said a statement issued by a group of local journalists, calling for more government protection.

Contributing to the drop-off in coverage is a pledge by some of Mexico's major news organizations to limit the publication of gruesome images to prevent cartel leaders using the media as a propaganda tool.

As mainstream media go quiet, scared citizens are turning to online sources for clues about how to stay safe.

The Mexican Internet Association (AMIPCI) says Internet use has doubled since 2005 to 35 million users, around 30 percent of the population. Twitter use has exploded to 2.5 million active users in March from just 32,000 in July 2009.

News of the dumping of 35 bodies in the eastern port city of Veracruz last week broke first on Twitter, as passers-by posted photos and warnings to avoid the area.

At the University of Saltillo in the northern state of Coahuila, students posted pictures on Twitter of a menacing banner signed by a drug cartel and strung up on the campus, and warned people to skip classes and avoid danger.

"There was nothing about the narco-banner at the University in Saltillo on the news. They never say anything, like always. There is nothing left to do but watch out for yourselves," said one message posted on Twitter last Friday.

Some social media users are taking note of the warnings.

"(Macias' killing) has a huge impact. The one thing the Internet has is freedom and now even there you have to be careful," said one Nuevo Laredo journalist who posts when shootouts happen in the city.

He also asked to remain anonymous.

"There is no point in denouncing something on the Internet if the authorities aren't going to do anything about it; you are only exposing yourself to more danger."