M . ▁B ren nan ▁Inc ., ▁who ▁helped ▁maintain ▁pl umb ing , ▁he ating , ▁air ▁condition ing ▁and ▁vent ilation ▁equipment ▁at ▁the ▁plant . ▁These ▁four ▁employees ▁noticed ▁prop ane ▁gas ▁pool ing ▁as ▁F alk ▁employees ▁started ▁the ▁prop ane ▁system , ▁which ▁the ▁factory ▁uses ▁as ▁a ▁backup ▁fuel ▁supply ▁when ▁natural ▁gas ▁is ▁shut ▁down ▁during ▁peak ▁demand . ▁These ▁employees ▁alert ed ▁other ▁workers ▁to ▁the ▁problem . ▁There ▁had ▁been ▁testing ▁conducted ▁on ▁the ▁system ▁for ▁a ▁full ▁ 4 0   min utes ▁prior ▁to ▁the ▁explos ion . ▁It ▁is ▁also ▁thought ▁the ▁leak ▁came ▁from ▁an ▁under ground ▁pipe ▁that ▁ran ▁from ▁prop ane ▁t anks ▁to ▁the ▁prop ane / air ▁mix ers , ▁which ▁convert ▁the ▁liquid ▁prop ane ▁into ▁g ase ous ▁form , ▁so ▁that ▁it ▁is ▁in ▁a ▁form ▁us able ▁by ▁the ▁bo ilers . ▁This ▁is ▁back ed ▁up ▁by ▁the ▁discovery ▁of ▁a ▁small ▁pocket ▁of ▁gas ▁tra pped ▁below ▁as ph alt ▁directly ▁below ▁the ▁Anne x ▁building . ▁Press ure ▁testing ▁of ▁the ▁pipe ▁using ▁nit ro gen ▁has ▁since ▁confirmed ▁it ▁was ▁le aking . ▁Sub sequ ent ▁exc av ation ▁showed ▁it ▁had ▁two ▁separate ▁le aks ; ▁ ▁Met all urg ists ▁have ▁exam ined ▁the ▁dam aged ▁sections ▁of ▁pipeline ▁and ▁determined ▁that ▁the ▁pipe ▁cor rod ed . ▁F alk ' s ▁ev acu ation ▁procedure ▁will ▁also ▁feature ▁prom in ently ▁in ▁the ▁investigation .