The bombs are duds. Meaning that even if they attempt to set them of, nothing will happen.
And, presumably the FBI is keeping so well tap on them that if they somehow got acces to real explosives (which can't be that hard, considering Tanerite is legal, as is fertilizer if purchased in small enough quantities) they would be arrested earlier.
They may be 'duds', but there are plenty of instances of the FBI providing actual explosives even if the 'detonator' is not going to work large quantities of high explosives in and of themselves are dangerous.
And, presumably the FBI is keeping so well tap on them that if they somehow got acces to real explosives (which can't be that hard, considering Tanerite is legal, as is fertilizer if purchased in small enough quantities) they would be arrested earlier.