The long-elusive Amari Saifi, a former Algerian special forces paratrooper known by his nom de guerre, Al Para, was absent from the courtroom on Saturday to the surprise of lawyers and journalists.
No explanation was given and his whereabouts remain a mystery.
Saifi was convicted by Algiers' criminal court for "constitution of a terrorist group" and for "propagating terror among a population".
Two of five other defendants, all present for the one-day trial, were sentenced to two years in prison. Three were acquitted.
The United States and European countries, particularly Germany, had pressed for the capture of Saifi.
He is considered the mastermind of the kidnapping in 2003 of 32 European tourists, mostly German, one of whom died of heatstroke, and also was wanted in the killings the same year of 43 Algerian soldiers.
Saifi is considered the Number Two in the Salafist Group for Call and Combat, GSPC, which declared allegiance to al-Qaida two years ago.
He remained elusive in court, although he was captured by Chadian rebels, then mysteriously turned over to Libya before being handed to Algerian police last autumn.
"For Algerian justice, Amari Saifi is not under detention"
Judge presiding over Saifi's trial
"For Algerian justice, Amari Saifi is not under detention," said the judge presiding over the trial in response to a lawyer's query.
The statement suggests the possibility that police never turned Saifi over to judicial authorities after his maximum 12 days of detention.
Saifi was captured by Chadian rebels who said they came across him and accomplices in the desert as the men wandered, lost, in northern Chad, in flight from West African armed forces.
He then fell into the hands of Libya after months of negotiations with various countries.
Libya turned "Al Para" over to Algeria in October 2004, the Algerian Interior Ministry said at the time.
The United States and European and African countries had pressed the Chad rebels for months to turn Saifi over to Algeria.
Germany had issued arrest warrants for Saifi and other GSPC leaders in connection with the 2003 kidnapping of 32 European tourists.
President Bouteflika says the
insurgency is almost defeated
Algerian commandos freed 14 of the captives, while reports said Germany paid a ransom for the remaining 17 who had been taken to neighbouring Mali. Germany refused to confirm the reports.
The GSPC, the most structured group among Algerian Islamic
insurgents battling the state since 1992, has in recent years turned its sights on jihad beyond Algerian borders.
Nabil Sahraoui, a GSPC leader who took over in 2003 before reportedly being killed by Algerian soldiers, declared the group's allegiance to al-Qaida.
The GSPC is a dissident faction of the Armed Islamic Group, now almost annihilated by Algerian security forces, and originally limited its targets to police, soldiers and others representing the state.
However, France has long seen the GSPC as a potential threat outside Algerian borders, including in France.
Around 150,000 people have
been killed in Algeria's civil war
The French daily Le Monde, in its weekend edition, quoted intelligence sources as saying that there are increasing signs the group is joining an "international jihad".
The paper said the group wrote a letter in October 2004 to Abu Musab al-Zarqawi, the Jordanian-born Islamist fighter.
In a more immediate threat, only weeks ago, GSPC fighters in trucks attacked an isolated Mauritanian army outpost near the Algerian and Mali borders.
The assault left 15 Mauritanian soldiers and nine Salafists dead.