buenos aires -- Thousands of protesters took to the streets here Tuesday to mark the one-year anniversary of the disappearance of Jorge Julio Lopez, a key witness in the successful prosecution of a police chief from the "dirty war" years.
Many hoisted banners bearing the rumpled likeness of Lopez, who was 78 when he vanished. His name and face have become emblematic of civil society's determination to prosecute abusers from the former military dictatorship that left thousands dead or missing.
"We are convinced that the disappearance of Jorge Julio Lopez is a message to spread fear, to make us forget what happened," said Enrique Fukman, a former political prisoner and protest organizer.
The media here have labeled Lopez as Argentina's first and only political desaparecido (disappeared) since the restoration of democracy in 1983 after seven years of military dictatorship.
Argentina, under the leadership of left-leaning President Nestor Kirchner, has gone further than most Latin American nations in pursuing former officials of the military juntas of the 1970s and 1980s.
Hundreds of ex-officers face trials after rollbacks of pardons and amnesties.
But the Lopez case underscored the vulnerability of witnesses, some torture victims themselves, whose testimony is important to bringing suspects to justice.
"The protection of witnesses is a crucial factor that has been missing," said Jose Miguel Vivanco, executive director of the Americas division of Human Rights Watch.
"The perpetrators of these crimes have enjoyed full protection until now, so they are going to do everything in their power to obstruct investigations," Vivanco said in a telephone interview from Washington.
Many suspect that junta loyalists snatched Lopez in retaliation for his testimony and to discourage prospective witnesses in other human rights cases.
"I don't doubt that Julio Lopez was kidnapped for revenge and as a warning to others," Pino Solanas, a film director and opposition presidential candidate, told Pagina 12 newspaper. "Behind all this is the residual apparatus of the dictatorship."
The government says it has augmented security for witnesses, providing escorts and other protections. But some have still reported threats. The specter of retaliation hangs over the trials.
Authorities say they continue to investigate the case of Lopez's disappearance. No remains have been found.
"I want to believe that he is alive, but I don't want to sound like a hypocrite," Ruben Lopez, his son, told reporters.
The elder Lopez was last seen Sept. 18, 2006, on a street near his home in La Plata, south of Buenos Aires.
A labor activist, Lopez was arrested during the dictatorship and interrogated with electric shocks, according to his testimony. He eventually was released and returned to a low-key life as a construction worker and family man.
However, relatives said he was determined to seek justice. The chance came last year. Lopez provided crucial testimony in the high-profile trial of Miguel Etchecolatz, a notorious former provincial police chief accused of taking sadistic delight in the torture and killing of detainees.
Among other things, Lopez linked Etchecolatz to the execution of a young activist couple shot while in custody at the lockup where Lopez was being held. The day after Lopez disappeared, Etchecolatz was convicted of illegal abduction and homicide. He is serving a life sentence.
Andrés D'Alessandro of The Times' Buenos Aires Bureau contributed to this report.