It was a grenade, and when it exploded in a small wooden barn near Dunkirk on 28 May 1940, it was the beginning of a massacre of more than 80 prisoners of war from the Royal Warwickshire and Cheshire Regiments by an SS regiment under Wilhelm Mohnke, one of Hitler's favourite soldiers.
The force of the explosion threw the dead bodies of two young soldiers on top of Mr Tombs, now 81. There were more grenades thrown among the prisoners who had been herded into the barn, then machine-gun fire. 'I heard shots, they were going round shooting people for quite some time. I just lay there still, covered in blood.'
Now that Germany has finally decided not to prosecute the man he believes gave the order for the massacre to begin, Mr Tombs is left with one ambition: 'I just hope I live to see him to his grave.' Mr Tombs was one of only a dozen men to escape from the barn, four of whom are still living.
After 15 minutes or more, he and four others decided to try to break out. At a farmhouse they were captured by a German soldier and feared they would be shot. 'But he was regular army, Wehrmacht, and said: 'You will go to Berlin, and we will be in London'.'
Mr Mohnke, now 82 and living near Hamburg after a successful business career, has been identified by witnesses, and named in Parliament by the Labour MP Jeff Rooker as the officer responsible for ordering the massacre at Wormhoudt, near Dunkirk, and at least two similar atrocities.
It was revealed this week that the German authorities have completed a four-year investigation into Mr Mohnke's war record, and the British government has yielded to pressure from Mr Rooker and others on behalf of the dwindling band of survivors, to release its secret 1947 files on the former Nazi general's massacre role.
Mr Mohnke was not considered for prosecution at the Nuremburg trials because he was captured by the Soviet Union at the end of the war. By the time he was released in the mid-1950s, his case and hundreds of others left on the files had effectively been forgotten by British authorities. He returned home, enjoyed a successful business career, and now lives on an army pension of more than pounds 20,000 a year.
Mr Tombs returned home from the war to work for the City of Birmingham bus company, and now lives in Droitwich with his wife on a pounds 92-a-week pension. 'Yes I'm bitter that he's living there with his army pension still. I always believed they'd get him in the end . . .'
Mr Tombs' three-page affidavit sworn to British Army investigating officers in London in 1947 will be among the documents released at the Public Record Office in the new year.