Three photojournalists who covered the perilous crime beat in the violence-torn eastern Mexico state of Veracruz were found slain and dumped in plastic bags in a canal on Thursday, less than a week after a reporter for an investigative newsmagazine was beaten and strangled in her home in the same state.

Press freedom groups said all three photographers had temporarily fled the state after receiving threats last year. The organizations called for immediate government action to halt a wave of attacks that has killed at least seven current and former reporters and photographers in Veracruz over the last 18 months.

Like most of those slain, the men found Thursday had been among the few journalists left working on crime-related stories in the state. Threats and killings have spawned an atmosphere of terror and self-censorship among the journalists of Veracruz, leaving most local media outlets too intimidated to report on drug-related violence. Social media and blogs are often the only outlets reporting on serious crime.

Mexico has become one of the world's most dangerous countries for journalists in recent years, with news people disproportionately targeted as a government offensive against drug cartels and rivalry among crime gangs have brought tens of thousands of killings, kidnappings and extortion cases.

Prosecutions in journalist killings are all but unknown, although it is generally the case with almost all homicides and other serious crimes in Mexico.

The latest killings came in Boca del Rio, a town near the port city of Veracruz where police found the bodies of four people Thursday after passers-by spotted four suspicious black plastic bags in a wastewater canal, the Veracruz state Attorney General's Office said. The bodies, pulled from the canal by police boats, bore signs of torture and had been dismembered, the prosecutor's office said.

One victim was identified as Guillermo Luna Varela, a crime-news photographer for the website www.veracruznews.com.mx who was last seen by local reporters covering a car accident Wednesday afternoon. According to a fellow journalist, who insisted on speaking anonymously out of fear, Luna was in his 20s and had begun his career working for the local newspaper Notiver.

Slain photojournalists Guillermo Luna Varela, left, and Gabriel Huge pose for a picture in Veracruz, Mexico.(Credit: AP)

The journalist said Luna was the nephew of another of Thursday's victims, Gabriel Huge. Huge was in his early 30s and worked as a photojournalist for Notiver until last summer, when he fled the state soon after two of the paper's reporters were slain in still-unsolved killings.

Huge had returned to work as a reporter in the state, but it was not immediately clear what kind of stories he was covering most recently.

State officials said the third victim was Esteban Rodriguez, who was a photographer for the local newspaper AZ until last summer, when he too quit and fled the state. He later came back, but took up work as a welder. The London-based press freedom group Article 19 said he, like the other two men, had been a crime photographer.

The fourth victim was Luna's girlfriend, Irasema Becerra, state prosecutors said.

Article 19 said in a report last year that Luna, Varela and Rodriguez were among 13 Veracruz journalists who fled their homes because of crime-related threats and official unwillingness to protect them or investigate the danger. The Committee to Protect Journalists said in 2008 that Huge had been detained and beaten by federal police when he tried to cover a fatal auto accident involving officers.

State officials said the killings of the four bore the hallmarks of organized crime and they would ask federal authorities to help investigate.

Veracruz is a common route for drugs and migrants coming from the south on the way up to the United States. Much of the area around the state's main port city on the Gulf of Mexico turned in recent years into a battleground between the Zetas cartel and New Generation, a gang based in the western state of Jalisco that is allied with the powerful Sinaloa cartel led by kingpin Joaquin "El Chapo" Guzman.

The Zetas are known for their ruthless targeting of anyone perceived as an opponent, and for their deep infiltration of local law enforcement in many Mexican states.

Last June, Miguel Angel Lopez Velasco, a columnist and editorial director for Notiver, was shot to death in Veracruz along with his wife and one of his children.

Authorities that month also found the body of journalist Noel Lopez buried in a clandestine grave in the town of Chinameca. Lopez, who disappeared three months earlier, had worked for the weeklies Horizonte and Noticias de Acayucan and for the daily newspaper La Verdad.

The following month, Yolanda Ordaz de la Cruz, a police reporter for Notiver, was found with her throat cut in the state.

Lopez was found after a suspect in another case confessed to killing him, but the other two murders have not been resolved.

The cartel war in Veracruz reached a bloody peak in September when 35 bodies were dumped on a main highway in rush-hour traffic. Local law enforcement in the state was considered so corrupt and infiltrated by the Zetas and other gangs that Mexico's federal government fired 800 officers and 300 administrative personnel in the city of Veracruz-Boca del Rio in December and sent in about 800 marines to patrol.

Mike O'Connor, the Committee to Protect Journalists' representative for Mexico, said journalists in Veracruz were exercising an unusual degree of self-censorship even before Ordaz and Lopez were killed. He said media avoided much coverage of crime and corruption.

"Important news was not covered because it might upset the Zetas. Then these guys were killed and self-censorship cracked down even more," O'Connor said. "Almost all of the police beat reporters left town after those killings."

Regina Martinez, a correspondent for the national magazine Proceso, continued to cover crime-related stories along with a handful of other journalists, however.

On Saturday, authorities went to her home in Xalapa, the state capital, after a neighbor reported it to be suspiciously quiet. They found the reporter dead in her bathtub with signs she had been beaten and strangled.

Veracruznews director Martin Lara said that last year Luna was frightened so badly he left the state and stopped working for Veracruznews for two months.

"He got a threat and he had to go," Lara said.

Lara declined to provide details. He said Luna was last seen Wednesday afternoon.

"Self-censorship was extraordinarily strong but whoever killed these journalists wanted more," O'Connor said. "It still wasn't enough to satisfy whoever killed these journalists."

Mexico's human rights commission says 74 media workers were slain from 2000 to 2011. The Committee to Protect Journalists says 51 were killed in that time. It noted in a statement on the Mexico killings that Thursday was World Press Freedom Day.