A small technical quirk in the engineering of the Ford Granada car used as the getaway vehicle by the Dalkey security van raiders may have helped to prevent, or at least to have delayed, the start of a renewed terrorist campaign on this island.

After ramming and immobilising the security van and threatening its crew and some passers-by, four raiders grabbed a dozen bags containing about £600,000 in cash and jumped into the Granada, which was waiting with a driver and with its engine running, ready to speed the gang and their money from the scene. The sudden impact of 12 heavy bags of cash in the boot and three men jumping into the rear seat is believed to have triggered a safety cut-off valve designed to stop petrol running to the engine in the event of an accident.

Witnesses saw the three raiders - each still carrying an AK47 assault rifle - get out and try to push the car. As they got out, the car seemed to start again, although with the engine running badly, and they jumped in again only, it seems, to trigger the cut-off once again and immobilise the car.

In desperation they seem to have pushed the car a few yards to the steep incline on Nerano Road, off Sorrento Road, where they had rammed the security van. The Granada rolled to the bottom of Nerano Road towards Dalkey Harbour but refused to take off up the slight hill on Coliemore Road.

At this point the gang were aware from the radio scanner they had in the Granada that gardai had left Dalkey station and were at the scene of the robbery only 200 yards behind them and that other gardai were almost certainly beginning to block the few escape routes from the village.

The gang panicked and left the bulk of the cash in the boot of the Granada, taking only one bag containing £80,000. They forced the driver of a passing red Saab 900 to stop and, in an act that must still make the Saab driver shudder, fired a shot through the back driverside window at his head. The bullet shattered on the metal frame of the headrest, and he received only a minor graze. It was, gardai concluded after examining the car, a remarkable escape.

The Saab was then driven to Glenageary, where it was switched for another waiting getaway vehicle. It was nearly six hours before the Saab was noticed in the grounds of St Paul's Church. The gang had escaped.

The gang had stolen the two commercial vehicles used in the robbery but had purchased the Granada through a newspaper advertisement. Ironically, it was a car formerly used by the Garda to escort cash deliveries. It was sold at an auction of Garda vehicles some years ago and had changed hands a few times since.

The gang's reason for buying such a car rather than stealing one was to attract as little attention as possible. A stolen car on the road is a liability even for a short time as it can be noticed by a quick-minded garda. An ageing saloon, apparently taxed and insured, attracts almost no attention. This practice of buying rather than stealing cars had been commonplace in the North since the 1980s. It was the favoured method of loyalist assassination gangs.

Two Isuzu vans, one a flat-back and the other with a standard container box, were used to stop and ram the security van. The ram consisted of two 18ft steel girders bolted together and running the length of the van and protruding about five feet from the rear. The driver's cab on the Isuzu was protected by more steel plating to ensure the girders went through the security van door and not through the Isuzu cab.

The first part of the heist went perfectly. The Brinks-Allied van was blocked at the junction of Sorrento and Nerano Roads, and the flat-back was reversed with pinpoint accuracy into the driver-side back door of the security van. One of two armed men from the Granada fired a shot into the rear wheel of the security van to immobilise it further and to show the crew that they were armed and ready to use their guns.

Including the driver of the second getaway car and a probable scout car which was following the security van, gardai reckon there were seven or eight men in the gang.

In assessing the robbery over the next few days, gardai began to come to some disturbing conclusions.

It had been meticulously planned; the gang had considerable resources; were well directed, with good intelligence on the movements of the security van; and were the most heavily armed robbers seen in this State since the mid-1980s, when the Provisional IRA was last involved in such large-scale robberies.

The location, between Dalkey and Killiney, was chosen probably because the leaders of the gang knew gardai would scramble their Aerial Wing immediately. But it would take the helicopter up to 15 minutes to arrive, and Killiney and Dalkey present difficult terrain for aerial surveillance, with hilly, twisting roads partially obscured by trees.

The area is also almost entirely residential and has one of the lowest suburban crime rates in the State, so it would be unlikely that local officers would be in a state of readiness for armed raids.

Other characteristics of the raid have also given gardai cause to worry. The precision and extent of the preparations suggest the involvement of people who could only have learned their trade with the Provisional IRA. The previous experiences of the Garda Special Branch in dealing with the dissident republicans in the so-called "Real" and Continuity IRA groups always led to the conclusion that they were dealing with people with limited resources and abilities who were not well led.

The only previous attempt to raid a security van by a dissident group, at Ashford, Co Wicklow, was compromised from the outset, and armed gardai were waiting for the gang. In the ensuing confrontation the gang was outgunned by the Garda's Emergency Response Unit (ERU), and one of the raiders, Ron an MacLochlainn, was shot dead. The Ashford raiders were armed with only one old AK47, two shotguns and a pistol. i.

The three members of the Dalkey gang seen pushing the Granada each had an AK47. Gardai found ammunition in the car which suggested a fourth raider had a powerful handgun. There was no doubt among senior gardai that the gang was that heavily armed for only one reason. They were ready to kill any gardai who happened to get in their way.

There appear to be differences of opinion between gardai about which paramilitary group was responsible, although all agree it was a paramilitary rather than "criminal" raid. While some senior gardai were quick to point the finger at the dissident groups, either the "Real IRA" or Continuity IRA, other officers were certain that from what they knew of these groups to date neither had the ability to stage such a large and well-planned raid.

The Garda Special Branch had considerable successes before the Omagh bombing last August in infiltrating and disrupting dissident republican operations. Gardai were well aware, through their information sources, of the identities and associations of most of the active dissident paramilitary figures. Most of the potentially dangerous republicans, particularly those associated with the "Real IRA", have been under surveillance since.

Also, after Omagh Government sources were quietly though confidently predicting that the dissident republicans were finished and that the raft of anti-terrorist legislation introduced by the Department of Justice in response to Omagh would be sufficient to deter the emergence of any new group. Much emphasis was also put on the supposedly deterrent effect on the dissidents of reported visits to their homes and threats from Provisional IRA figures who support the ceasefire.

Dalkey undermines the case that the dissidents are finished. Here was an expertly planned raid executed by terrorists who were quite prepared to kill anyone who got in their way. According to some gardai, the raid has shown not only that these "dissidents" are undeterred by the shame of Omagh and the so-called "draconian" anti-terrorist laws and threats from the Provisionals, but that they appear to be determined to continue and even escalate their activities.

Even more worryingly, there is also evidence at Dalkey to show that the group responsible is attracting figures who must have been among the Provisional IRA's most able and dangerous figures.