The cartel has already attacked rivals, journalists and other perceived enemies. Now, the target is an online chat room, Nuevo Laredo en Vivo, that allows users to comment on the activities of the Zetas and others in the city on the border with Texas.

Already, three apparent site users have been slain, and a fourth victim may have been discovered Wednesday, when a man’s decapitated body was found with what residents said was a banner suggesting he was killed for posting on the site. Chat room users said they could not immediately confirm the victim’s identity, because people all post under aliases.

Despite such precautions, users are highly vulnerable, and the Zetas could be tracking them from clues they leave online, experts said Thursday.

A female chat room user was found decapitated in September with a similar message as the one found Wednesday and at the exact same spot, with a message signed with the letter “Z,” which refers to the Zetas. Residents couldn’t fully read the latest message, because the dead man’s body was laid on top of it, in what appeared to be a more hurried execution.

Drug cartels in Mexico have frequently attacked traditional print newspapers, by tossing explosives at their offices or killing, kidnapping or threatening reporters. Violence against journalists in Tamaulipas state, where Nuevo Laredo is located, has led local media to censor themselves, leaving residents on their own to separate fact from pervasive rumors spread on social networks.

Juan Carlos Romero, who helps lead the press freedom group Article 19, said local newspapers have often stopped publishing crime reports out of fear, leading residents to turn more to the Internet for information like that posted Thursday on Nuevo Laredo en Vivo: where gunshots have been heard, where vehicles suspected of carrying cartel lookouts have been seen, which streets are safe to travel.

“What are people doing in the face of the lack of information, the kind of information you need to make decisions: Where can I drive? Can I leave the house?” said Romero. “People are forging new channels of communication on the Internet, social networks, Twitter, blogs, Facebook.”

Drug cartels appear to have learned that such Internet sites reach far more readers than northeastern Mexico’s small regional newspapers and have adjusted their attacks accordingly.

“We are witnessing a new behavior of criminal forces in the country,” said Erick Fernandez, a communications professor at the IberoAmerican University in Mexico City. “We are in a new phase.”

Romero agreed. “It appears to me that organized crime is trying to get common citizens to stop real-time coverage of violence,” he said, saying that “the intimidation is having a multiplier effect.”