An explosion in an LNG storage tank in the process of being repaired could have occurred as the result of static electricity, malfunctioning electrical equipment, an open flame in contact with the liner, or sabotage. Engineers estimate that such an explosion would occur with probability of .25 given static electricity, .20 given malfunctioning electrical equipment, .40 given an open flame, and .75 given sabotage. Safety and security specialists estimate that the prior probabilities of static electricity, malfunctioning electrical equipment, open flame, and sabotage to be .30, .40, .15, and .15 respectively. Based on this information, what was the most likely cause of the explosion?