Gunmen on a motorcycle assassinated a government prosecutor investigating one of Colombia's bloodiest paramilitary massacres in years, and a U.S. human rights group said Saturday that two of her colleagues are missing and feared dead.

Yolanda Paternina, 50, was shot twice as she was returning home from work Wednesday in the city of Sincelejo, 330 miles northwest of the capital, Bogota, according to police.

Two investigators working with Paternina on the case disappeared in June and are feared dead, said Robin Kirk of Human Rights Watch.

The three were probing allegations of state complicity in a massacre in which dozens of paramilitary gunmen hacked to death 26 people in the northern village of Chengue after accusing them of collaborating with leftist guerrillas.

Even though residents in the region pleaded for protection from the paramilitary army months before the January massacre, authorities failed to prevent the killings.

On Friday, Human Rights Watch sent a letter to Colombian Attorney General Luis Osorio urging him to investigate the crimes and to protect the prosecutor now heading the investigation.

The massacre raised fresh doubts about the government's willingness to rein in the paramilitary United Self-Defense Forces of Colombia, or AUC. Curbing paramilitary violence is a key condition for continuation of military aid and training from the United States under a $1.3 billion regional anti-drug package.

The 8,000-strong AUC is responsible for the most of the human rights atrocities committed in Colombia's 37-year civil war.

Meanwhile Saturday, the Revolutionary Armed Forces of Colombia — the nation's largest leftist rebel army — freed a journalist with local Radionet radio station they had kidnapped in March. In an interview after his release, 68-year-old Guillermo Angulo told the station where he works as a foreign news editor that no ransom was paid.