a large gas pipeline exploded into a tower of fire on friday in central california .
the explosion and fire closed both directions of a major highway and injuring at least 15 people , four of them critically .
it was not clear what caused the explosion at the fresno county sheriff 's gun range that brought traffic in the area to a halt .
traffic heading north and south on highway 99 in fresno was halted the explosion about 2.30 pm as flames towered over the roadway .