The US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is offering a reward of up to $50,000 for information leading to the arrest of persons or persons who deliberately set fire to a West Fertilizer Company plant in the town of West, Texas, in April 2013. After a long silence punctuated only sporadically during the three-year investigation that cost an estimated $2 million, ATF officials said on May 11 they had concluded that the fire and subsequent explosion that killed 15 people – most of them firefighters – and injured more than 260 others was deliberately set and thus a criminal act.

AFF said its verdict followed extensive scientific testing and more than 400 interviews as well as a review of witness photos and videos that had eliminated all accidental and natural fire scenarios.

The conclusion was that the blaze originated in the seed building of the fertilizer plant. Reports said the explosion – registering as a 2.1-magnitude earthquake – occurred 14 minutes after the first call to emergency responders. In its wake, about half of the town’s 700 homes clustered around the plant, including a nursing home, were affected and many of them destroyed or severely damaged.

Previously, the investigating law enforcement agencies had said they were considering three possible scenarios, including faulty electrical wiring in the production facility, a short-circuit in an electrical golf cart and an intentional act of arson.

The Chemical Safety Board (CSB), the US federal agency that investigates chemical disasters and recommends new safety standards, said this was “one of the most destructive episodes” it had ever investigated.

In particular, CSB said it was concerned about the storage of ammonium nitrate within the plant. While saying it had long been known that the fertilizer active ingredient has explosive properties under certain conditions, it added that “this was poorly understood and communicated to the public” in the small agricultural community where farmers have used the company’s products without incident.

A year after the blast the board criticized the plant’s management for not informing the locality’s volunteer firefighters that the compound was stored there, even though reports sent to other state agencies as a legal obligation revealed that in 2012 the facility had 540,000 lbs of ammonium nitrate stored.

CSB said West Fertilizer had failed to take the necessary steps to avert “a preventable fire and explosion” and that federal, state and local regulatory agencies had failed to identify a serious hazard and correct it.

US media said the ATF findings may be introduced into the many pending lawsuits facing the now defunct West Fertilizer and could also trigger new lawsuits. The explosion is said to have caused some $230 million in losses.

Defendants in the litigation include plant owner Adair Grain Co., El Dorado Chemical Co., CF Industries, Thermacline Inc. and International Chemical Co. – all of whom are said to have manufactured starting materials for fertilizer and sold them to West.  Adair reportedly has countersued the other defendants. Several of the plaintiffs have settled out of court on undisclosed terms.