A Mexican man whose quest for justice on behalf of his kidnapped son led him to join a national peace movement was fatally shot in the northern city of Hermosillo, authorities said Tuesday.

Nepomuceno Moreno Nuñez, 56, was killed around midday Monday as he was driving through the city in a pickup truck, the Sonora state police said.

Investigators found seven shell casings at the scene.

Moreno left jail 18 months ago after being acquitted on charges of carrying illegal weapons and shooting up a house in 2005, the state police said.

He spent most of the last year of his life seeking to draw attention to the July 2010 abduction of his son and four other people in Ciudad Obregon, a crime he attributed to state police.

Two of the four other abductees remain missing, while one was found dead in a canal and the other was freed by his captors.

Authorities, who said they found 18 cloned bank debit cards inside the vehicle in which the five young men were traveling, said the kidnapping was linked to organized crime.

Moreno took part in May in a March for Peace led by poet and journalist Javier Sicilia, whose son was brutally slain in Cuernavaca, a resort city near the Mexican capital.

Nepomuceno Moreno was also among the people invited by Sicilia's Movement for Peace with Justice and Dignity, or MPJD, to participate last month in a televised encounter with President Felipe Calderon.

Moreno recounted his son's story and told Calderon he had received death threats for his outspoken advocacy.

All the evidence indicates Moreno's murder was linked to organized crime, the victim's own criminal record and his son's abduction, Sinaloa Attorney General's Office spokesman Jose Larrinaga told a press conference Tuesday.

He said Moreno was convicted in the United States in 1979 of conspiracy to smuggle heroin.

Moreover, Larrinaga said, one of the people abducted along with Moreno's son was facing car-theft charges and another was involved in a Sept. 2008 homicide in Ciudad Obregon.

Moreno is the second MPJD member to be murdered, following the Oct. 7 slaying of Pedro Leiva Dominguez in the western state of Michoacan.

MPJD activists held their own press conference to condemn Moreno's killing and demand justice.

It is "unworthy and shameful" for the Sonora AG's office to blame Moreno for his death, the MPJD's Pietro Ameglio said, denouncing "a climate which reeks of fascism."

He said authorities jailed Moreno's other son on "completely invented charges" and that the dead man's daughter was fired from her civil service job because of her father's activism.