the explosion at the fresno county sherrif 's gun range happened on a pacific gas & electric co pipe carrying natural gas .
an equipment operator and a group of jail inmates were expanding a road alongside highway 99 , which was closed after explosion for three hours .
ten inmates and the operator were hospitalized , three of whom were in critical condition .
three inmates were evaluated and sent back to jail and two deputies were being evaluated for ringing ears and exposure to the hot blast .
california public utilities commission said it is investigating the explosion .