Almost a month into their review of the deadly blast at a Texas fertilizer plant, investigators are hoping to draw a picture from the air of how the plant looked before the explosion and compare it to the 93-foot-wide crater that's there now.
They'll paint and mark off lines for the walls of each building at West Fertilizer, where an April 17 explosion killed at least 14 people. Then, they'll fly overhead to compare the lines to the crater.
"At the end, we're going to be able to look back down from an aerial view and say, 'OK, here's where our building was,' in reference to where the crater was," said Brian Hoback, a national response team supervisor for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
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That process will occur as investigators try to reconstruct whatever they can of the plant -- from electrical wires to pieces of walls -- in hopes of nailing down how ammonium nitrate detonated in a blast that sent debris flying through ceilings blocks away and registered as a small earthquake.
Their work has taken longer than expected, causing some frustration as people continue to wait for answers. After saying they might finish this week, officials now say they'll need more time and are asking for patience.
"We're not going to leave anything unturned on the scene, and there just happened to be more that needed to be turned," assistant state fire marshal Kelly Kistner said. "That's all there is to it."
State and federal workers have sorted through much of the debris with rakes, shovels and by hand. The material they considered to be possible evidence was stacked atop blue tarps -- "boneyards" -- scattered over the site. The rest of the debris was trucked away.
A site that was covered in twisted metal and planks of wood last week had been cleared in most parts by Tuesday.
They had also slightly expanded the wall of one building rebuilt through pieces of debris and were planning to re-position power lines in their original locations on the site.
"We're not talking about a 100 percent reconstruction. You can't do that, especially at this site. That's not going to happen," Hoback said. "But what you try to do is you try to reconstruct those things that are important to you in terms of origin and cause."
While the fire marshal's office said this week that stores of ammonium nitrate exploded, officials said they still didn't know what caused it to detonate or what caused the fire beforehand. The explosion occurred about 20 minutes after authorities were alerted to the fire.
"The No. 1 issue that we're trying to resolve right now is, where did the fire start and how did the fire start?" Kristner said.
Kistner laid out four possible factors, any one of which could have caused the ammonium nitrate to explode: heat, possibly from the fire; a physical shock to the chemical; an issue with how it was contained or stored; and contamination.
"So at this point, we all know we had a fire and we had heat, right?" Kistner said. "If we look at the shock, the containment and the contamination, those are things we're still looking at and still investigating."
Officials still are investigating how much ammonium nitrate was on site and whether any other chemicals were involved.
The large crater left in the blast has been mapped, excavated with heavy equipment and raked through by investigators, Hoback said. It could provide clues to how much ammonium nitrate was on site and other details of the blast, officials have said.
"We're looking for artifacts of the building, our cause, and our origin," Hoback said.
NBC 5's Eric King contributed to this report.