(Ethan Horton/Staff Writer)Federal investigators have determined that the response to a blaze at Robbins Lumber on May 15 likely inadvertently contributed to the subsequent explosion that has killed three people and severely injured more. The result was an explosion of dust that launched the silo and a large plume of fire into the air. Investigators said Robbins Lumber employees were unaware that the silo’s contents — wood shavings and sawdust — constituted a combustible dust hazard. The facility had hosted walk-throughs for firefighters, the report said, but there was no “pre-incident plan” with site-specific strategies. To save lives, investigators want new rules requiring bold signs that say: “DANGER — Sealed Silo — Water Contributes to Explosion” on facilities including silos, bins, tanks and hoppers.