Source: Luck of the Devil by Ian Kershaw.
Comment: Gestapo believed there was English involvement in assassination attempts on Adolf Hitler in 1939 (but no evidence survived the war). This is just to record some sources regarding the use of English munitions (which is generally accepted) in the July 20 plot.
SS Report on the Conspiracy, 26 July 1944
Reich Chief Security Office - IV -
Special Commission for July 20, 1944. Berlin, July 26, 1944
Report on the plot against the Fuehrer of July 20, 1944.
On July 20, 1944, at about 12:50 PM, a detonation occured in the 'Wolfsschanze,' Restricted Area A, visitors' barrack, during the briefing session. The Fuehrer suffered only slight injuries, although in the immediate vicinity of the centre of the explosion...
Immediately after plot became known, Reich Leader SS appointed speical commission of Reich Main Security Office for investigation of the attempt. Investigations began on the same day.
Reich Leader SS noted during detailing of circumstances that perpetrator of attempt was presumably Graf von Stauffenberg, colonel, chief of staff at office of replacement army. He had been present at briefing session, then withdrew without notice before the explosion. Immediately afterward he went to Berlin by plane.
Specific place of the assassination attempt was conference room where daily briefing sessions took place, about 40 feet long and 15 feet wide. In the centre of the room stands a large map table; to the left, writing desk and radio phonograph. Room and all furnishings heavily damaged. To the right of entrance, 22 inch hole in floor. For wider radius, floor pressed in and charred. Points of impact of metal fragments not detectable, but wood splinters and leather fragments are impacted into fibre-board walls.
Bomb crater shows that explosion occurred above surface of floor. Reconsituted fragments of the right section of three sections of table show clearly the direction of the pressure wave. It is indicated in photographs and sketches.
Lower pressure wave from detonation continued through cavities under floor through the entire barracks and caused minor damage to a brick and cement framework of the barracks, particularly by a buckling upward of the floor. Upper pressure wave destroyed conference room to a large extent and found exit through window and door, as well as through partition wall. Very minute sifting of mass of rubble has led to discovery of extremely small leather and metal fragments, obviously from a brief-case; of pieces of sheet metal and two compression springs from English chemical-mechanical time-fuse firing pins; also, part of flat iron pliers. Other material discovered has no obvious connection with the explosive.
Along the road leading from south exit to airport, an engineer search unit has found: a 976-g lump of explosive, with priming chambers of 20 g each and one English chemical-mechanical time-fuse firing pin, connected to English detonator cap, and set for 30-minute delay. Explosive was wrapped in brown wrapping paper.
Medical report: Injuries and contusions attributed solely to pressure of blast. Additionally, considerable burns. In several wounds, fragments of wood, straw and, presumably, tattered leather. X-ray photographs reveal in all only two small metal spinters, presumably from lock of brief case.
Leather fragments that were discoverd have been identified by witnesses as belonging to Stauffenberg's briefcase. Small parts of igniter found at place of explosion come from two igniters that are of same type as the two English chemical-mechanical time-fuse pins found along the road. As two compression springs from this type of time-fuse were found at place of explosion, the explosive charge must have contained two such igniters. The charge that was found along the road was also arranged for two igniters. Therefore the explosive that was used for the attempt was presumably of exactly the same kind as that which was found. According to report of explosive expert, the extent of damage at place of explosion correponds to potential power of explosive that was discovered.
Driver of car that took Stauffenberg to airport observed that he threw an object out the window in the general area of the discovery, and driver has given an affidavit. Thus Stuaffenber's complicity has been objectively ascertained.
On 6 July, Stauffenberg was present, for the first time in his capacity as chief of staff to Fromm, at two hour-long briefings at the Berghof. He had explosives with him. But, it seems, an appropriate opportunity did not present itself. Whatever the reason, at any rate, he made no attempt on this occasion. Impatient to act, Stauffenberg resolved to try at his next visit to the Berghof, five days later. But the absence of Himmler, whom the conspirators wanted to eliminate along with Hitler deterred him. Again, nothing happned/ on 15 July, when he was once more at Fuehrer's Headquarters (now moved back to the Wolf's Lair in East Prussia), Stauffenberg was determined to act. Once more, nothing happened. Most probably, it seems, he had been unable to set the charge in time for the first of the three briefings that afternoon. While the second short briefing was taking palce, he was telephoning Berlin to clarify whether he should in any case go through with the attempt in the absence of Himmler. And during the third briefing, he was himself directly involved in the presentation, which deprived him of all possibility of priming the bomb and carrying out the attack. This time, Olbricht even issued the 'Valkyrie' order. It had to be passed off as a practice alarm-drill. The error could not be repeated. Next time, the issue of the 'Valkyrie' order could not go out ahead of the assassination attempt. It would have to wait for Stauffenberg's confirmation that Hitler was dead. After the bungling of the opportunity on the 15th, the third time that he had taken such a high risk to no avail, Stauffenberg prepared for what he told his fellow conspirators, gathered at his home in Berlin's Wannsee district on the evening of 16 July, would be a last attempt. This would take place during his next visit to the Wolf's Lair, in the briefing scheduled for 20 July.
Already, as the Stalingrad crisis deepened towards the end of 1942, Tresckow - later described by the Gestapo as 'without doubt one of the driving-forces and the "evil spirit" of the putschist circles', and allegedly referred to by Stauffenberg as his 'guiding master' (Lehrmeister) - was pressing for the assassination of Hitler without delay. He had become convinced that nothing could be expected of the top military leadership in initiating a coup. 'They would only follow an order,' was his view. He took it upon himself to provide the 'ignition' (Initialzuendung)', as the conspirators labelled the assassination of Hitler that would lead to their removal of the Nazi leadership and takeover of the state. Tresckow had already in the summer of 1942 commissioned Gersdorff with the task of obtaining suitable explosives. The latter acquired and tested various devices, including British explosives intended for sabotage and for the French resistance that had been captured following an ill-fated commando expedition to St Nazaire and a catastrophic assault on Dieppe in 1942. Eventually, he and Tresckow settled on a small British magnetic device, a 'clam' (or type of adhesive mine) about the size of a book, ideal for sabotage and easy to conceal. Olbricht, meanwhile, coordinated the links with the other conspirators in Berlin and laid the groundwork for a coup to take place in March. The plans to occupy important civilian and military positions in Berlin and other major cities were, in essence, along the lines that were to be followed in July 1944.
[Hitler wore bullet proof vest and hat]. Nor was the chosen sharpshooter, bearer of the Iron Cross with Oak Leaves Lieutenant-Colonel Georg Freiherr von Boeselager, sure that he was mentally equipped to shoot down a person - even Hitler - in cold blood. It was an entirely different proposition, he felt, from firing at an anonymouse enemy in war.
Nevertheless, Boeselager made preparations for a group of officers, who had declared themselves ready to do so, to shoot Hitler ona visit which, it was hoped, he would soon pay to Army Group Centre headquarter at Smolensk. The visit eventually took place on 13 march. The plan to shoot him in the mess of Field-Marshal von Kluge, commander of Army Group Centre, was abandoned since there was a distinct possibility of Kluge and other senior officers being killed alongside Hitler. Given Kluge's wavering and two-faced attitude towards the conspiracy against Hilter, more cynical plotters might have thought the risk well worthwhile. As it was, they took the vieew that the loss of Kluge and other leading personnel from Army Group Centre would seriously weaken still futher the shaky eastern front. The idea shifted to shooting Hitler as he walked the short distance back to his car from headquarters. But having infiltrated the security cordon around him and set up position to open fire, the assassination squad failed to carry out their plan. Whether this was because Hitler took a different route back to his car, or whether - the more likely explanation - the danger of killing Kluge and other officers from the Group was seen as too great, is unclear.
Tresckow reverted to the original plan to blow up Hitler. During the meal at which, had the origina plans been carried out, Hitler would have been shot, Tresckow asked one of the Fuehrer's entourage, Lieutenant-Colonel Heinz Brandt, travelling in Hitler's plane, to take back a package for him to Colonel Hellmuth Stieff in Army High Command. This was in itself nothing unusual. Packages were often sent to and from the front by personal delivery when transport happened to be available. Tresckow said it was part of a bet with Stieff. The package looked like two bottles of cognac. It was, in fact, two parts of the British clam-bomb that Tresckow had put together.
Schlabrendorff carried the package to the aerodrome and gave it to Brandt just as he was climbing into Hitler's Condor ready for take-off. Moments before, Schlabrendorf had pressed the fuse capsule to activate the detonator, set for thirty minutes. It could be expected that Hitler would be blown from the skies shortly before the plane reahed Minks. Schlabrendorff returned as quickly as possible to headquarters and infomred the Berlin opposition in the Abwehr that the 'ignition' for the coup had been undertaken. But no news came of an explosion. The tension among Tresckow's group was palpable. Hours later, they heard that Hitler had landed safely at Rastenburg. Schlabrendorff gave the code-word through to Berlin that the attempt had failed.