In the morning darkness last Wednesday, two buses made the 30-mile journey from the Tigantourine gas complex along a desert road to the airfield near the town of In Amenas in southern Algeria. A police escort led the gas workers being ferried to catch their flight, a quiet reminder of the potential terrorist threat in the Sahara Desert which stretches out to the south of the plant.

The first bullets that announced an attack hit the buses at about 5am. Passengers threw themselves to the floor as guards and police returned fire at more than 20 gunmen in vehicles who had waited to ambush them. Two men, a Briton and an Algerian, thought to be one of the security guards, were unable to take cover in time and both died. They were the first victims of North Africa's largest foreign hostage-taking drama in recent memory.

At Tigantourine itself, a sprawling gas works on the northern edge of the Sahara, the usual morning routines were interrupted soon after. Many of the 132 foreign and 570 local workers were still at the residential camp, a grid of trailers and tents two miles from the main gascomplex. Others, such as Alan Wright, had gone to work early that day at his office near the refinery. The 37-year-old health and safety worker from Portsoy, Aberdeenshire, was interrupted when the power went off and an alarm sounded. It was a fairly normal occurrence, he said, so along with a handful of colleagues he went to the usual muster point. Seeing no one else there, he returned to his office. It was then he was told Tigantourine was under attack: "We locked ourselves in the office, taped papers over the window so no one could see inside and sat tight," he said. "There was gunfire outside, sometimes intense, and then at times quiet and we sat for three-and-a-half hours. Other colleagues told us the terrorists wore either military uniform or gendarmes, so we would not know who was friend or foe."

In a canteen in the residential area, workers having breakfast were plunged into darkness before men shouting "God is great" burst in on them.

"All of a sudden there were bursts of fire, explosions," said an Algerian employee. "They were smashing the doors, shouting 'We're looking for foreigners - you Algerians can go'." The expats they found were divided from locals and tied up in a dining hall.

Security sources have indicated that the militants may already have had men on the inside working as cooks or guards. All over the complex scenes of confusion and panic were repeated.Some workers hid, others fled. In many cases, decisions made in the first minutes of the seizure decided whether they survived. Alexandre Berceaux , a Frenchman, elected to leave people he was with, lock himself in his room and hide under the bed. He stayed there for 40 hours until colleagues convinced him to open the door to Algerian soldiers who had come to rescue him.

Others were killed for showing the same level of trust. An Algerian cook told how on the first day of the siege he saw an unnamed British man with a gun put to his head who was forced to call out to other colleagues hiding in the complex: "They threatened him until he called out to his friends in English: 'Come out. Come out. They're looking for Americans. They're not going to kill you.' A few minutes later they blew him away."

Three Britons in one of the dining halls climbed into the compartment above a suspended ceiling, where they tried to remain silent for three days.

As Wednesday turned to Thursday and the gunmen of the al-Qa'ida splinter group carried out a door-to-door search of the refinery and the residential area, other fighters planted mines and booby-traps ahead of anticipated assault by the hundreds of Algerian troops, tanks and special forces known as "Ninjas" they knew to be massing in the desert beyond a cordon six miles from the perimeter of the plant.

According to witnesses, including Mr Wright, who made a dash for the fence at first light on Thursday morning, shots were already being fired. The Algerian military struck with full force in mid-morning when militants appeared to be trying to flee into the Sahara in a convoy of 4x4 vehicles with at least 30 of the hostages, who were forced to wear "bomb necklaces".

The convoy was strafed from the air by Algerian Hind helicopter gunships, which fire 5,000 rounds a minute. Four of the vehicles were obliterated, with piles of incinerated bodies found later by security forces. The Algerian troops, marshalled by General Arthman Tartag - who had a reputation for brutality during the country's savage civil war - launched a full ground assault and retook the residential area.

The surviving terrorists, still holding an unknown number of hostages, retreated by the end of Thursday to the refinery itself to make their last stand. In a series of calls to a radio station in Mauritania, they said they were holding seven foreign hostages. A series of demands followed, including a cessation of Western operations in Mali and the release of jailed militants. All were refused. Fires were then lit on Friday night amid reports that the hostage-takers would try to blow up the plant.

Those fires were doused, and after a tense all-night standoff, a final assault was launched on the factory on Saturday morning. It is not known if any of the hostages were rescued in that firefight. Seven were executed as Algerian troops stormed the buildings still being held, bringing the death toll to 23 hostages, with 32 of their captors also killed.

Today, as bomb disposal crews and heavily armed detachments scoured Tigantourine, those numbers were already rising. The full toll of the Algeria hostage crisis and siege may take several more days to emerge.

How the siege unfolded

Wednesday

Shortly before dawn, heavily armed gunmen in vehicles storm the Amenas gas field complex, in Algeria's east, taking a group of international workers hostage. Algerian Interior Minister Daho Ould Kabila says the kidnappers are operating under the orders of Mokhtar Belmokhtar, who was a senior commander of al-Qa'ida in the Islamic Maghreb (AQIM) until late last year, when he set up his own armed group. The militants reportedly ask to leave the country with the hostages, but Mr Kabila later says he refused to let them go.

Thursday

The Algerian military launches a surprise attack using helicopters, shooting indiscriminately at vehicles inside the compound driven by the kidnappers but which are also carrying about 35 hostages. Conflicting media reports say anything between a dozen and 30 hostages are killed, as well as 11 to 18 militants. Some 600 hostages are freed however, including about half of the foreign workers.

Associated Press reports that the kidnappers want to exchange two American hostages for two jailed terrorists. Later that day, it is reported that the accommodation area, where hostages were being held has now been secured but that "six or seven terrorists" have taken refuge in the industrial part of the plant, which is encircled by Algerian security forces.

Saturday

Algerian forces launch a dramatic final assault on the plant to end the siege and capture the remaining kidnappers. Just beforehand, the captors kill seven of the remaining hostages, but 16 are freed. The remaining 11 kidnappers are killed in the raid. Sonatrach, the state-run gas and oil company, says troops have discovered that the plant had been mined.

UK Defence Secretary Philip Hammond calls the loss of life appalling and unacceptable. He says "it is the terrorists that bear the sole responsibility for it". President Barack Obama says the US stands ready to provide whatever assistance Algerian officials need in the aftermath of the attack.