(CNN) -- Two journalists kidnapped earlier this week by armed gunmen in Gomez Palacio, Mexico, were freed by their captors unharmed Saturday, according to the state-run news agency Notimex.
Mulitmedios cameraman Javier Canales and Televisa Laguna photojournalist Alejandro Hernandez were released two days after the release of another hostage, Hector Gordo, a journalist on the program Punto de Partida.
The whereabouts of a fourth hostage, Oscar Solis, a local newspaper reporter, are still unknown.
The journalists were kidnapped Monday while covering a protest by inmates and relatives at a local prison. The prison made news after Mexico's attorney general's office revealed some of its guards let a squad of imprisoned hit men free to carry out a massacre of 17 people in the nearby town of Torreon.
This week, local residents along with Mexican and foreign journalists started a petition on the social media sites Facebook and Twitter to demand the journalists' release.
On Friday, the signal of one of Mexico's largest television networks faded to black for almost an hour as a symbolic protest of violence against journalists.
"We will not pretend that nothing is happening," said Denise Maerker, anchor of Televisa's "Punto de Partida," as she opened the show.
The protest Friday comes after the four journalists were kidnapped Monday while covering a riot at a prison in the northern state of Durango.
Earlier news reports indicated that an unidentified drug cartel has demanded coverage of videos it has made in exchange for releasing the four reporters, the New York-based Committee to Protect Journalists said in a statement Wednesday.
The journalists were abducted in the Laguna region, which includes the state of Durango and parts of neighboring Coahuila state, Mexico's National Commission for Human Rights said in a statement Tuesday.
That area has been the scene of vicious fighting between the Zetas crime group and the Sinaloa cartel.
Violence against journalists has become increasingly common amid the escalating drug war, and critics have said the government does not do enough to hold criminals accountable.
"Most crimes against reporters remain unsolved. Authorities rarely determine who perpetrated the crime and there are no prosecutions much less convictions," Mexican journalist Dolia Estevez wrote in a May working paper published by the Mexico Institute at the Woodrow Wilson International Center for Scholars.
On Friday, Mexico City's Milenio newspaper published an editorial calling on the government to step up its game to solve the hostage situation. In the editorial, journalist Ciro Gomez Leyva alluded to demands from the kidnappers for the media to publish certain images in return for the release of the journalists.
"That is not our duty. The media is not designed nor equipped to negotiate in a hostage situation," Gomez wrote.
Pascal Beltran Del Rio, editorial director of one of Mexico's oldest newspapers, Excelsior, said Televisa's decision to fade to black "called the attention of authorities and the public to something happening out of the ordinary affecting the life and security of journalists."
Another reminder of that danger came late Friday when an explosion rocked the parking lot of the Televisa station in Nuevo Laredo. There were no reported injuries. Police said the target might have been the station's transmission tower.
Televisa last used the symbolic black screen in 1994 to mourn the death of presidential candidate Luis Donaldo Colosio, who was assassinated while campaigning in Tijuana.
CNN's Krupskaia Alis and Nick Valencia contributed to this report.