They had sniffer dogs, gas masks and came from Chechnya. From inside Middle School No 1, they watched the siege on television and rang accomplices from mobile phones who told them what was going on around the building.
Yesterday investigators and eyewitnesses gave a detailed picture of how 30 highly organised militants launched the events which claimed the lives of up to 400 people and left 562 injured in hospital, according to local officials.
Mr Sergei Ignatchenko, spokesman for the Russian security services, the FSB, said in a telephone interview: "They were very well trained. They had gas masks, as they had learned lessons from the Moscow theatre siege [where Russian special forces released a knockout gas into the auditorium to end the standoff]. They planted mines in every corridor. They brought with them two sniffer dogs."
He said there were about 30 gunmen, assisted by observers outside the school. "They stood in the crowd wearing civilian clothes, and told them what was happening. The terrorists came from Chechnya, that we know." He said it was unclear where they had crossed into North Ossetia.
Ms Margarita Komoyeva (44), a physics teacher at the school who was held with her daughters Diana ( 14), Alina (11) and Madina (3), was released with Madina in the first batch of hostages on Thursday. She said the militants took a TV set from the head teacher's office and watched coverage of the attack. "From TV reportage they understood that hostages in the gym were making phone calls and again forced us to hand over our phones." Another eyewitness said they shot a man for making a call.
Ms Komoyeva said: "Later one of the Chechens ran inside shouting, 'They are saying there are only 300 hostages here. This is because they want there to be fewer victims announced when they storm. If they don't care about you, then we won't either. We will fight and when we run out of ammo, blow both you and us away.'"
Investigators have struggled to work out how the militants got so much explosives into the school so quickly, leading many to suspect that some of the explosives had been hidden in the school during renovations over the summer. A regional education official, speaking on condition of anonymity, said one set of renovations, on the main school building, was done by a state-linked body. However Ms Komoyeva said other repairs had been carried out by a local company, but she did not know who.
It also emerged yesterday that the shooting began when a bomb inside the gym went off by accident, triggering an attack by local men in an armed militia.
The former president of neighbouring Ingushetia, Mr Ruslan Aushev, a negotiator, said he was in contact with Chechens until the last moment on telephone.
He told the newspaper Novaya Gazeta: "We sent people to collect the 21 dead. We agreed that a lorry with five doctors would come. When they arrived something happened at this school and a bomb went off."
At that moment the shooting began. "I called the \ to ask them to stop shooting. They said: 'We have stopped, it is your side.' But the official troops were not shooting, the terrorists were not shooting, and we are screaming at each other, 'Who is?' The men in the school said: 'That's it, the storm has begun, we will explode the bombs'."
He said he told them the elite Alfa unit was not storming the school. "We [in the crisis headquarters] ordered any shooting to be stopped, but a foolish third force - some kind of people's militia - appeared. They had AK-47s and decided to free the hostages. We are trying to investigate how all this happened."