The timing was not local. Through the autumn of 1962 the Cuban missile crisis was pushing the two blocs toward a war neither had planned, and under Soviet pressure the Polish authorities staged large exercises across the north-west of the country, in the country around Szczecin. Marshal Andrei Grechko of the Soviet Union came to watch, along with Wladyslaw Gomulka and Marian Spychalski. The parade was to be the finale, and Grechko wanted the best hardware the Warsaw Pact could field put on the street. The city was decked in Polish, Soviet and East German flags. The newspapers ran headlines to match - a parade of our might, a symbol of brotherhood in arms that serves the peace, Polish Szczecin welcomes allied forces. None of this was aimed at children in particular. It simply worked out that a demonstration staged for a Soviet marshal at the height of a nuclear standoff was also a free afternoon out.

It came down the avenues in order: East German equipment first, then Soviet, then Polish units, among them fourteen tanks of the 5th Armoured Division from Slubice on the Oder. Cannons, mortars, howitzers, armoured personnel carriers. Heavy Mi-6 helicopters passed over the rooftops, and Soviet warships lay moored in the port. The crowd kept edging forward, and the militia telling people to move back were ignored - a normal enough thing at a parade, until the vehicles are thirty-six tonnes and moving at thirty kilometres an hour. Why tank number 0165, a T-54 and the last in its line, went into the group of children near Aleja Piastow has never been established. It may have been going too fast. The road was slippery, and it may have skidded. Sources cannot even agree whether the children it struck were on the pavement or in the street, because within days the documents that would have settled it were being gathered up.

They were between six and twelve years old. Four of them were pupils at Szczecin's Elementary School Number 1, and the others came from schools across the city that had all been let out early for the same reason. Twenty-one more people were hurt in the impact, several of them children, some with injuries they carried for the rest of their lives. Then the crowd broke. In the panic that followed - thousands of people trying to get away from something most of them had not seen - another twenty-two were injured. Their names are not in the public record, and that is not an accident of history. The parents who went home that evening without their children were visited and told not to discuss what had happened with anyone. Whatever grieving those families did, they did quietly, in a city where everybody had been at the parade and nobody was permitted to say so.

On 10 October the local daily Glos Szczecinski carried a short notice stating that seven children had died during a parade. That was the sum of what Poles were told. The military prosecutor's office called in hundreds of witnesses and informed each of them that the matter was classified and that speaking about it would mean prison. Every hospital record connected to the day was confiscated by the military and never returned. The tank crew were never tried: the military court found that the tank had not crossed the kerb, and the case was dropped - a finding that answered the legal question by declining to ask the human one. In March 1982, twenty years on and with martial law in force, the remaining documentation was destroyed outright. What the authorities could not do was clear the pavements of everyone who had been standing on them. By the middle of October 1962 the city was full of rumours, most of them putting the death toll far higher than seven.

Aleja Piastow today is an ordinary Szczecin avenue - tram wires, plane trees, the tall pre-war facades that survived the bombing, students walking to the university buildings nearby. There is no monument on the spot. The commemoration that exists is small and specific: a tablet placed in 2006 at the entrance to Elementary School Number 1, for the four of its pupils who did not come back that afternoon. It took forty-four years and the end of the government that had suppressed the story. What remains after the confiscated hospital files and the destroyed prosecutor's records is a set of numbers - seven dead, twenty-one injured, twenty-two more hurt in the crush - and the knowledge that the state had gone to some trouble to get children to that kerb in the first place, with flowers in their hands, for homework.