Smaller bomb was used to draw revellers on to street

The deadly precision of Saturday's attacks in Bali was described yesterday. The drivers of two vans had stopped and blocked the narrow street where revellers were enjoying a night out at the Sari Club and Paddy's Irish Bar.

A small bomb exploded near Paddy's, which drew people outside – allowing a subsequent massive blast a few seconds later on the opposite side of the street to wreak maximum casualties. That is the scenario being painted by investigators in Bali, where a local newspaper – quoting police and intelligence sources – reported yesterday that eight men drove off in one van, leaving the second vehicle behind with its deadly cargo. According to the daily Koran Tempo, the main bomb was detonated by remote control or a timer. Indonesian police identified the vehicle carrying the larger charge as a Mitsubishi L-300.

Four days after an explosion destroyed the two nightclubs, killing more than 180 people, the multinational investigation is taking a variety of twists and turns. Yesterday, two Indonesian men were formally detained as suspects, while police acknowledged questioning a former Indonesian air force lieutenant-colonel with an expertise in explosives. Officials said the officer was not a suspect.

In Malaysia, authorities arrested four men linked to Jamaah Islamiya, the Indonesian militant Islamic group suspected by some of launching the bomb attack. At least one of the men was also believed to be connected with al-Qa'ida.

One source close to the Bali investigation said bombers used a mixture of plastic explosives and gas cylinders to maximise the death toll. The cylinders have been blamed for a fireball that followed the blast. The C-4 military explosives were probably packed into the van's roof, said a police spokesman, Saleh Saaf.

According to Tempo, the two vehicles used in the attack stopped in Legian Street, the main thoroughfare in the Kuta tourist area, causing a traffic jam behind and clearing a space in front. The occupants of the second van switched to the first, which sped off just before the bomb exploded.

The newspaper cited an intelligence official as saying the attack – which is being investigated by local police backed by officers from Australia, Britain and the FBI – was the work of seven Indonesians under the leadership of a person from the Middle East.

But some witnesses told Indonesian police they saw a man leave a white plastic bag at the scene and then flee shortly before the blast. The two Indonesians detained as suspects are a security guard and the brother of a man whose identification card was found at the site. One was in the Kuta area at the time of the bomb. The pair, who have been questioned intensely since Monday, are not from Bali.

Mr Saaf said: "We're afraid they might run away, so they had to be detained. They have caused difficulties related to the investigation." Police denied a report that a former air force officer had confessed to assembling the bomb but said they had questioned the man, who lives near the scene. They said the man, discharged from the air force last year for misconduct, went three times to the US to be given explosives training.

Alexander Downer, the Australian Foreign Minister, announced that Indonesia and Australia had agreed to form a joint intelligence and investigation team "to work through the issues flowing from the tragedy in Bali". Mr Downer, visiting Jakarta, said officials "still don't have any hard evidence as to who is responsible" for the Kuta explosion.

The Indonesian government is preparing to enact emergency anti-terrorism measures, to demonstrate to the international community its commitment to crack down on extremists. President Megawati Sukarnoputri plans to issue a decree to allow her to bypass parliament.

While Indonesia has been accused of allowing militant Islamic groups to flourish, Malaysia and Singapore have rounded up dozens of members of Jamaah Islamiya.

Western nations have repeatedly urged Indonesia to arrest the group's leader, Abu Bakar Bashir, who runs an Islamic boarding school in central Java. He has denied any involvement and yesterday blamed the bomb on non- Muslims who engineered it "to launch war against Islam".

* Rohan Gunaratna, an author and expert on al-Qa'ida, has cited documents from a US interrogation of a member of the group, which said an unidentified Saudi once sent $74,000 (£47,000) to Jamaah Islamiya to buy explosives. The information was gathered at interrogations of Omar al-Faruq, a Kuwaiti arrested in Indonesia in June.