听力原文： Police in London now say that more than 50 people died in yesterday's terrorist bombings and warn that the death toll could rise. Police also said that there were no more than four explosive devices involved, three on underground trains and one on a bus, and they estimated that all the bombs were relatively small containing about 10 pounds of high explosive. Commissioner Blair also denied there was any evidence to suggest that the attacks were carried out by suicide bombers but said the possibility could not be ruled out. Police said they received no warning about the attacks and would not speculate on a claim by a group calling itself 'The Secret Organization of Al-Qaida in Europe' that it was behind the bombings. We know for sure that ______.