The bodies of a man and a woman were found hanging from a pedestrian overpass in the Mexican border city of Nuevo Laredo this week with notes threatening similar action against other “Internet snitches.”

The gruesome display appeared to mark a move by drug cartels, which have murdered journalists for their reporting, to apply the same deadly pressure to any Mexicans who share information online.

Borderland Beat, a blog that tracks the Mexican drug war, posted photos of the notes as well as a disturbing image of the two bodies as they appeared to motorists before being cut down by the authorities.

The two were killed, CNN reported, for messages they had posted on well-known Internet sites that collect reports of drug violence in areas of the country where professional journalists are no longer able to safely do their jobs. As The New York Times’s Randal C. Archibold has reported, intimidation by drug traffickers has silenced many news organizations, especially along the border. As a result, local residents have tried to fill the information gap by using social media and a few bold news sites that cover the drug war like Frontera al Rojo Vivo and Blog Del Narco.

The two sites were specifically named in the notes, which were translated by Borderland Beat. “This happened for snitching on Frontera Al Rojo Vivo,” read a note attached to the man’s leg. Another, on the overpass, said:

This will happen to all the Internet snitches (Frontera al Rojo Vivo, Blog Del Narco, or Denuncia Ciudadano). Be warned, we’ve got our eye on you. Signed, Z.

The signature was an apparent reference to the Zetas drug cartel, according to CNN’s Mariano Castillo, and “denuncia ciudadano” is a form of anonymous report to the government that can be filed online.

The murders were all the more disturbing because, absent regular news reports on the drug violence, many in Mexico turn to Twitter and other social media for information. Hashtags — which tie Twitter posts together — have become an important sorting mechanism, turning connected reports by individual Twitter accounts into an ad hoc news service.

Andrés Monroy-Hernández, a doctoral candidate from Mexico at the Massachusetts Institute of Technology Media Lab, wrote in the Mexican policy magazine Nexos that one of the first cities to do this sort of bundling of news reports was eastern border town of Reynosa, creating a #reynosafollow hashtag.

Others soon copies the idea, including Monterrey (#mtyfollow) and Veracruz (#verfollow). In his analysis and visualization of more than 250,000 messages posted with these kinds of hashtags, Mr. Monroy-Hernández found, not surprisingly, that the dominant theme was “balaceras” — shootings.

Andrés Monroy-Hernández/Nexos Magazine

Asked about the reliability of the Twitter reports he analyzed, Mr. Monroy-Hernández responded in an e-mail:

I would say that in Monterrey and Saltillo, which is the two cities I have followed more closely, most of the information is reliable and the information that is not often goes ignored. The three “hub accounts” that I mentioned in my article (@trackmty, @AnaRent, @cicmty) serve as curators and do a decent job at it. Of course, there is false information too, but it has not been as prevalent. Again, it is really hard to make a scientific assessment since we never know what the ground truth is. But I have seen a correlation between Twitter spikes and what is reported in the local media. Another interesting thing is that a lot of the tweets are actually retweets (almost 50 percent). This tells me that people are engaged and cooperating with one another in spreading the information, but it also worries me a bit because I assume people often retweet without being 100 percent sure.

Indeed, reports of violence based solely on social media also open the door for rumors to spread quickly in the absence of reliable sources to knock down bad information.

“Mexicans who are using social media to navigate the chaos of the drug war increasingly find themselves between a rock and a hard place,” Nicholas T. Goodbody, who teaches Mexican cultural studies at Williams College, wrote in an e-mail. “Because they can become the target of criminal violence or government prosecution.”

In Veracruz, a man and a woman were charged with terrorism and sabotage after passing along rumors of an impending drug cartel attack on a school using the #verfollow tag on Twitter.

The Mexican authorities have accused the two, Gilberto Martinez Vera and Maria de Jesus Bravo Pagola, of inciting a panic and several accidents as parents rushed to remove their children from the school. Their lawyer told The Los Angeles Times that the government was seeking to make an example of the pair — Ms. Bravo is a well-known local journalist — in an effort to stem a tide of misinformation online.

Human rights groups and free-speech advocates said the charges for the so-called Twitter Terrorists were unjustified. Amnesty International wrote that the drug war “creates a climate of distrust in which rumors circulate on social media as people try to protect themselves, because there is no reliable information available.” While such information can be untrue, the group said, “It is the responsibility of the authorities to disseminate correct and reliable information about the measures it is taking to protect the population.”

The Los Angeles Times reported that the panic might have started more than an hour before the first Twitter message was sent.

Many have emerged to declare solidarity with the two Twitter users, declaring “Yo también soy #TwitTerrorista.”

Between the murderous cartels and fearful authorities, Mexican social media users who are hungry for accurate news about the drug war find themselves caught in a dangerous place where information is hard to come by, hard to check and potentially deadly to pass along.