The gunmen set up their faux barrage (fake checkpoint) at Beni Ounif, on the desert highway parallel to the Moroccan border on Saturday evening.
They flagged down cars and proceeded to slash the throats of 23 men, women and children. Six more people were shot dead as they tried to run away.
In Algiers, newspapers reported that the killers kidnapped two pretty 15-year-old girls and stole the belongings of their victims.
Months had passed since Algeria experienced such a massacre. A former foreign minister, Mr Abdelaziz Bouteflika, became president on April 15th and he had hypnotised his own population and foreign diplomats with a constant refrain of "peace, peace, peace".
On July 5th, Algeria's independence day, Mr Bouteflika announced a pardon for 5,000 imprisoned Islamist sympathisers.
The massacre at Beni Ounif jolted the country back to reality, to the fact that more than 400 people have been killed since Mr Bouteflika came to office.
The slaughter bore the markings of Mr Antar Zouabri, the leader of the Armed Islamic Group (GIA) whose death has been reported several times. Mr Zouabri has broken with Mr Hassan Hattab and Mr Abdel Madjid Dichou, two other GIA leaders now based in the mountainous Kabylie region east of Algiers.
For Mr Zouabri, every subject of the Algerian government is an "infidel" who deserves to be killed; Mr Hattab and Mr Dichou prefer to target soldiers and policemen. Earlier this month, they attacked the police commissariat in Tizi Ouzou, the provincial capital of Kabylie, in broad daylight.
The sheer number of armed men in Algeria is making it difficult to end the war. The government has given weapons to 300,000 "patriots" or militiamen allied with 200,000 soldiers, police and gendarmes.
The 4,000-strong Islamic Salvation Army (AIS), the armed wing of the Islamic Salvation Front (FIS), surrendered to the government last June after nearly five years of negotiations. The AIS remains encamped in the countryside and is used by the army to watch and attack the GIA.
"A lot of people have no interest in the resumption of peace," Mr Miloud Brahimi, a lawyer, said in a telephone interview from Algiers. "It's surprising to find that many of them work for the government."
Fewer than half of the prisoners whom Mr Bouteflika pardoned in July have been freed.
"We are told that there are people who cannot be freed because their sentences haven't been confirmed yet," Mr Brahimi said. "They have to be convicted before they can be liberated. It's Kafka - or Bokassa - take your pick."
The goal of the July 5th amnesty and of a scheduled September 16th referendum on a "law on civil accord" was to persuade the GIA to stop fighting. The rebels are being asked to disarm and return home under probation - with no guarantee they will not be tried.
"The guerrillas realised that Bouteflika is a clever diplomat who seduced them with promises that were not what they seemed," an Algerian journalist, who covers security matters, said. "They decided it was better to step up the violence in the hope of obtaining a better deal."
The extremists may no longer have the strength to carry out massacres on the scale of two years ago. But any peace will be fragile at best.
"Seven and a half years have passed and 100,000 people have died," the Algerian journalist said. "For nothing. People who supported the Islamists realise it hasn't worked. It doesn't mean they support the government; they may wait and rebel again later. It will be a sort of hibernation."
