High-explosive mortars fired from the car park of the Excelsior Hotel breached the airport's perimeter and landed on the apron of one of the two main runways. The subsequent closure of the runway was the first carried out in response to a terrorist action.
Mike Roberts, managing director of Heathrow Airport, admitted that it was almost impossible to protect and police more than nine-and-a- half miles of perimeter fence. 'We have examined the trajectory and we don't believe there is a fence we could build that could have stopped the attack,' he said.
Mr Roberts and leading policemen defended their decision to keep the airport open during the 93 minutes from the first warning until the northern runway was closed. This was how the drama unfolded:
5.07pm: The first of six news agencies received a coded warning about bombs at Heathrow. They included the words 'Stop all flights, clear all runways' but said a large number of bombs would go off within the next hour 'in' Heathrow.
Tom Roche, of Sky Television, denied an IRA claim that the warning mentioned 'mortar bombs trained on Heathrow airport'.
Paul Condon, Commissioner of the Metropolitan Police, said the Anti-Terrorist Branch, police at Heathrow and BAA, the former (and privatised) British Airport Authority consulted and decided not to close the airport. 'I totally endorse that decision.'
He said that there was a grave danger of the media playing into the hands of the Provisional IRA. 'One of their prizes would be to disrupt normal life and the transport system. I believe the media almost handed them that prize this morning.'
5.16pm: A routine security procedure was launched. Mr Roberts said the passenger terminals and runways were 'swept' for anything suspicious following the warning. Runway inspection was carried out by vehicles equipped with searchlights and specialised sensors. Nothing was found. 'We get quite a number of bomb threats,' he said. 'This particular one was non-specific. They are very difficult to handle. The problem with evacuation is that unless you are sure the device is in a certain area you can evacuate people into the face of danger.'
5.57pm: The police received the first indication of a fire in the Excelsior Hotel car park. Mr Condon said the fire was an 'inferno' involving at least six cars and it took police officers 10 minutes to get close enough to establish that there was a mortar plate inside. There had been no reports of a mortar attack. He added that had the 20,000 people in the airport terminals been evacuated, there was a chance some would have been moved to the car park.
Four mortars from a stolen red Nissan Micra car, registration number A274 TGK, had been fired from the car park, over Heathrow police station and the perimeter fence and landed on a 20-metre wide apron at the edge of the 45-metre wide north runway. None exploded, although traces of high explosive, including Semtex, found later indicate that the damage could have been extensive.
Aircraft continued to use the runway for take off for 43 minutes until debris was spotted during a search.
6.35pm: Decision was made to close the runway. This order took about five minutes to execute.
A further search found two mortar-fired rockets - cylinders approximately 45cm (18ins) long, 6cms (2.25ins) in diameter with fins on one end. The search continued through the night and a third was found in grass alongside the runway.
The car used to fire the mortars was stolen in Kilburn, north-west London, last Saturday evening. It carried the genuine number plate A274 TGK but false plates, A568 MTM were fitted before the explosion. Police are anxious to trace its movements. Information will be treated in confidence on the special terrorist hotline, 0800 789321.
Police are also examing security videos shot in the car park but said that the coverage was not extensive.