I also seem to remember witnesses and audio tape describing three separate explosions. The pond draining explanation of that was that what people heard and machines recorded were merely echo's of one single explosion.
The seismograph at the university of OK picked up two events. This was explained away as a refraction at first, then as the explosion, followed by the building's collapse. Do a search on the OK City Bombing at
http://www.thenewamerican.com for info, this has been around since the dust started settling. The JBS smelled a rat right away.