HOUSTON (AP) — A series of failures, including flawed equipment and inadequate safeguards, helped cause a 2014 poisonous gas leak that killed four workers at a Houston-area chemical plant, according to a federal agency's final report on the deadly accident.

Four employees at the now-closed DuPont chemical plant in LaPorte, Texas, died in the release of methyl mercaptan — a chemical used in the manufacture of insecticide and fungicide.

The chemical began leaking from a valve around 4 a.m. on Nov. 15, 2014, in a unit at the plant in La Porte, about 20 miles (32.19 kilometers) east of Houston. Ki...