First, my deepest condolences to all families and friends of dead and injured in this explosion. Apparently, some facts need further clarification. April 19th is a critical date in US history, since, whereas siege at Waco, TX ended on that date probably by coincidence (ATF went in on that particular day after 50 days of siege, negotiating and waiting), it was Tim McVeigh who "comemoraded" that day (and the deadly siege) two years later by bombing federal building in Oklahoma City. To do that he used masive amounts of fertilizer as an explosive. Fertilizer itself is inert, but under certain conditions it may explode. In any case after that bombing sale of fertilizer is severly restricded and I suppose non-farmers are not even capable of obtaining big amount of it. So, if somone wants to make a huge explosion by fertilizer the only other option (apart from stealing it) is to set it on fire wherever it may be. Either some stock on some farm, or directly in some fertilizer factory. I surely hope that this explosion is caused by an accident. However it's up to authorities to determine how did this happend, and possibly why the affected area had not been evacuated after initial fire broke out.