By Daniel Schweimler
BBC News, Buenos Aires
Tens of thousands of people marched through the centre of the Argentine capital calling for more be done to find a missing "Dirty War" witness.
Human rights activists led the protests
Julio Lopez was a key witness at the trial of a former police chief on human rights crimes committed under military rule in the 1970s and 80s.
Mr Lopez, 77, disappeared the day before the sentencing.
Most marchers fear the worst - that he was kidnapped and killed by supporters of the 1976-83 military government.
He was tortured by agents working for that government and gave key evidence in the trial of former police chief Miguel Etchecolatz, who was sentenced to life imprisonment last week for kidnap, torture and murder.
An estimated 30,000 people were kidnapped and killed under military rule.
They became known as "the disappeared" since most of their bodies have never been found.
Now human rights activists are talking about Julio Lopez being the first person to disappear since the return of democracy in 1983.
They have reason to be pessimistic. Several witnesses in the trial were threatened, with recordings of torture victims being played down their telephones, and the three judges hearing the case were also threatened.
Mr Lopez may have gone into hiding
It could also be that Mr Lopez, a 77-year-old former labourer who suffers from Parkinson's disease, felt the pressure of the high-profile court case and simply wandered off.
A huge police search has been launched and a $70,000 reward offered for information on his whereabouts.
The first civilian governments after military rule passed laws which allowed the perpetrators of what became known as the "Dirty War" to walk free.
The Argentine Supreme Court last year ruled those laws to be unconstitutional and the trials began again, with Miguel Etchecolatz being the highest profile figure to be sentenced so far.
Justice in Argentina is again being seen to be done, but it is proving to be a painful process.