It could have been a been a boiler explosion or a steam line rupture. I say this because I experienced a steam line rupture at the power plant I use to work at. The damage that the line caused was very great. Twisted I-beams, buckled deck plates/grating ect. Unfortunately it also blew down a door to the control room and killed many of co-workers. The steam at pressure and temp that that line was running expanded by volume I was told to around 1600 times..very destructive. They said if it would have been the main steam line it would have flattened the whole plant. Steam explosion is a very good candidate for the second explosion. Look what happened to the S.S. Sultana. But like you said probably never be known 100%.