Having taken just a cursory look, there are dozens of stories about a fire in a mechanical room at the library. Some say they heard what sounded like an explosion. Yet no one saw anything but smoke.
It sounds to me that a pump or compressor blew out.
Most people have never heard a true explosion. They would definite report the noise of an electrical or mechanical failure as an “explosion.” First reports are almost never reliable.
Of course, this is a classic Intel diversion tactic that played out that day. Weren’t the Bros recruited by CIA or was it FBI? (Can’t recall). In any case, the JFK library was in smoke and flames and tweeters heard an explosion; the place was evacuated and the archives, for sure, were effected. Sad day for JFK librarians.