Mexico City (CNN) -- A high-profile Mexican peace movement blamed the country's government Tuesday for the death of one of the group's most vocal members.
Nepomuceno Moreno Nunez, 56, was shot Monday afternoon as he crossed a street in Hermosillo, Sonora, authorities said.
Investigators collected nine shell casings from the scene.
"For the death of Nepomuceno, we blame, by omission, federal and state authorities who did not respond to his requests for protection," the Movement for Peace with Justice and Dignity said in a statement issued Tuesday.
His death "is a message not only against the movement, but also against everyone who works in a civil and pacifist manner," Pietro Ameglio, a member of the movement, told reporters.
Moreno Nunez participated in a public meeting between members of the movement and Mexican President Felipe Calderon in October. Other peace activists were present at the meeting, including poet Javier Sicilia, one of the movement's leaders and a vocal critic of the Mexican government's war against the drug cartels.
"I am shocked, angry and outraged," Sicilia said in an interview with Milenio Television after Moreno Nunez's killing. "I am speechless. Protective measures had been requested for (Moreno Nunez), who had received threats. He himself asked for protection."
Moreno Nunez was outspoken in his search for justice in the kidnapping of his son and three others in Sonora. He had accused police of "accosting" the youths and having a hand in their disappearance.
"It seems that here everyone knows what happens, except for the police," Moreno Nunez said in a recently recorded video posted on the peace movement's website. "I am searching for what has not been given to me in my state: justice, comfort, respect."
The Sonora state attorney general's office denied that the activist had requested protection before his death.
The office did not say whether Moreno Nunez's killing was related to his work as an activist, but did describe his criminal history to reporters. The office said that Moreno Nunez had been imprisoned previously for possessing a weapon exclusively for military use and that he was involved in an armed incursion of a home in 2005.
According to the Mexico City daily Reforma, however, the authorities failed to mention that he was released from prison after being absolved.
Sonora attorney general's office spokesman Jose Larrinaga Talamantes also noted that Moreno Nunez was convicted in the United States on drug trafficking charges in 1979.
Authorities suspect his slaying was connected with organized crime, he said.
The Movement for Peace with Justice and Dignity called on state authorities to investigate the killings thoroughly and to provide protection to Moreno Nunez's family as needed.
"We have not stopped investigating the disappearance of the victim's son ... and this will be part of the homicide investigation," Larrinaga said.
Nearly 43,000 people have been killed in drug-related violence since Calderon took office in December 2006, according to the U.S. Drug Enforcement Administration.
CNNMexico's Javier Rodriguez and Jose Luis Rivas contributed to this report.