Mexican journalists will protest Thursday to mark one month since the killing of their colleague Javier Valdez, a noted expert on drug trafficking whose murder looks a long way from being solved.

Valdez, 50, was shot a dozen times in broad daylight on May 15 outside the offices of Riodoce, the newspaper he founded in the violent western state of Sinaloa.

More than 100 journalists have been killed in Mexico since 2000. But the death of Valdez -- a veteran crime reporter, award-winning author of several books and longtime contributor to AFP -- proved to be a tipping point.

His killing drew international condemnation and calls for justice from Mexican journalists.

The surge of outrage forced President Enrique Pena Nieto to publicly address the issue of journalists' murders for the first time, vowing to reinforce protective measures and do more to bring those responsible to justice.

More than 90 percent of journalists' murders remain unsolved in Mexico.

Valdez was at least the fifth reporter killed so far this year. Another, Salvador Adame, the head of a local TV station in the state of Michoacan, has been missing since gunmen abducted him on May 18.

Mexico is the third-deadliest country in the world for journalists, after Syria and Afghanistan, according to the watchdog group Reporters Without Borders.

Many of the victims were reporting on the country's multi-billion-dollar drug cartels, rampant government corruption, or both.

- Opaque investigation -

One month on, the investigation into Valdez's death shows little progress.

His family members say the authorities have kept them in the dark, and the special unit created to investigate crimes against journalists declined to speak with AFP about the case.

State prosecutors have refused to comply with a freedom of information request, citing a new law that protects certain information such as suspects' identities.

They initially said they were investigating the case as an auto robbery before acknowledging Valdez's journalism to have been the likely motive.

"There is evidence, there are clues," lead prosecutor Ricardo Sanchez insisted Wednesday in an interview with the radio station Formula, declining to give further details because the investigation remains open.

Valdez's colleagues aren't convinced -- especially since the government offered an $85,000 reward Tuesday for tips on the case.

Valdez had recently been reporting on a war between rival factions of the powerful Sinaloa drug cartel since its boss, Joaquin "El Chapo" Guzman, was extradited to the United States in January.

Colleagues at Riodoce suspect he may have been killed in retaliation for an interview he published with the head of one faction, Damaso Lopez Nunez, who is battling El Chapo's sons for control of the cartel.

Riodoce -- where Valdez was beloved for his charisma, dark humor and sense of solidarity with fellow journalists -- has called for protests Thursday in Culiacan, the state capital.

Protests and public debates are also planned in Mexico City and other cities around the country.

Foreign correspondents have appealed to international media outlets to publish stories about Valdez on Thursday and spread the word about the case using the social media hashtag #OurVoiceIsOurStrength (#NuestraVozEsNuestraFuerza, in Spanish).