The blast killed twelve of the first responders at the scene, and three others elsewhere. It left a crater 93 feet wide and ten feet deep. Debris was scattered over a 37-block area, and pieces were found 2.5 miles away. Over 104 people from the two agencies spent 30 days going over the scene of the blast, excavating and mapping the crater, generating nearly 300 leads, interviewing 500 people. They combed an area covering about 14 acres. At one point, investigators looking for evidence sifted through an entire silo of corn — 300,000 pounds — by hand.

Despite that effort, what caused the fire isn't clear. Investigators have rules out a number of causes. It wasn't due to an earlier fire rekindling, as had been speculated. It wasn't spontaneous ignition from equipment in the plant or its 480-volt electrical system. It wasn't weather or smoking.

There are three possibilities. The first is that a second electrical system in the plant, its 120-volt system, malfunctioned. The second is that a battery failed on a golf cart kept in the room where the fire started. There have been cases in which such failures have started fires, and investigators have only found two small pieces of the cart, impairing their ability to evaluate it.

And then, of course, there's arson. The investigators pointed out that their investigation led them to Bryce Reed, an EMS worker arrested for possession of bomb-making materials. "At this time," a representative of the ATF told reporters, "authorities will not speculate whether that possession has any connection" to the explosion at the fertilizer plant. Beyond that, there was no indication of who might have intentionally set the fire.

The investigation in West, at the scene, is over. Now the investigation, which remains open, turns largely to analyzing the evidence they already have.

This article is from the archive of our partner The Wire.