Several hours before the surrender, at either 11:30 or 2:15 the Irish Public Record Office (PRO) block located in the western block of the Four Courts, which had been used as an ammunition store by the Four Courts garrison, was the centre of a huge explosion, destroying Irish state records going back to the Norman conquest. Forty advancing Free State troops were badly injured. Assigning blame for the explosion remains controversial. It was alleged by the National Army Headquarters that the Anti-treaty forces deliberately booby-trapped the PRO to kill advancing Free State troops. Tim Healy, a government supporter, later alleged that the explosion was the result of land mines laid before the surrender, which exploded after the surrender. However, a study of the battle concluded that the explosion was caused by fires ignited by the shelling of the Four Courts, which eventually reached two truck loads of gelignite in the munitions factory. A towering mushroom cloud rose 200 feet over the Four Courts. Calton Younger identified three explosions: "two beneath the Records Office at about 2.15 and another at the back of the building at about 5 o'clock".