I wondered about that, too. Can TATP be used that way? The articles have said that he was not successful in buying the fertilizer, but perhaps there was somebody else who was successful and they were planning on putting these triggers on larger explosive devices that they had already put in place (although it says the stadium had been swept for bombs, so presumably anything larger would have been found).
There have been thefts of 7,500 pounds of fertilizer in CA, 2000 pounds of jet fuel in Kentucky and a fire in a self-storage unit...fertilizer was discovered.
Wonder about other thefts or purchases in other states of fertilizer...