WEST, Texas — Investigators working to figure out what caused a massive, deadly fertilizer plant explosion in Texas have talked to more than 370 people and received more than 200 tips as they continue to search for a breakthrough.

Two weeks after the April 17 blast that killed at least 14 people, agents compare their work to solving a puzzle or completing an archaeological dig.

“We’re trying to find the critical piece,” said Chris Connealy, the state fire marshal, on Thursday.

Their work is complex for several reasons: the magnitude of the blast at West Fertilizer, which knocked out windows and rooftops all over the tiny town of West and registered as a small earthquake; the deaths of 10 first responders and two others who volunteered to help; and the spread of debris as far as two miles away.

Agents are using digital mapping of the plant, rakes, shovels and front-end loaders to sift through dirt and rubble over an approximately 15-acre site. Possible bits of evidence are being cataloged and tested.

Investigators in dark blue uniforms and light blue helmets raked through piles of dirt and lifted debris. Many of them had worked almost every day since the blast. Small skid-steer loaders carried away piles of dirt that already had been examined.

On a cool, windy day, the faint smell of fertilizer filled the air, and officials said the air quality was continuously being monitored. Remnants of burned-out cars and trash littered the sides of the plant site.

Some of the remaining wall from a plant building had been raised at the site, as officials also are trying to reconstruct as much of the plant as possible as part of their investigation. A specialist from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives will later design a model of what the plant looked like at the time of the explosion, so that authorities can test several scenarios of what happened, said Assistant State Fire Marshal Kelly Kistner and Robert Champion, ATF’s Dallas special agent in charge.

Authorities have not yet begun to fully investigate the approximately 90-foot-wide crater left by the blast, Kistner said. On Thursday, teams were sifting through dirt at the former site of an administrative building next to the crater.

Chemical tests so far have only revealed fertilizer at the site, Kistner said. He said he didn’t know yet what specific chemicals were found or how much ammonium nitrate was on site during the blast.

Two months before the explosion, it reported the capacity to store as much as 270 tons of ammonium nitrate, though how much was actually on site when the blast occurred is unknown. In addition to mapping the site, agents have spoken to the plant’s owner and are reviewing documents to find out how much ammonium nitrate was there, Kistner said. They have previously ruled out a rail car carrying ammonium nitrate.

“This is planting season. This is fertilizing season,” Kistner said. “This is a busy, busy time for this facility right here. So they were constantly turning product, tons a day, out of this facility.”

Officials would not reveal all of the technology they are using, but said the process could continue after a previously set May 10 target date. They also continue to investigate the possibility that the blast was a criminal act.

“Nothing has been ruled out at this point,” Champion said.

Officials defended the amount of time they were taking to finish the investigation, saying they owed it to the victims of the blast to take their time, and criticized speculation about the blast. Kistner criticized speculation about the explosion as uninformed.

“It’s not becoming a distraction to the investigation, but I think it’s a distraction to the public and to the people, and the families of the people, who lost their lives here,” Kistner said. Because you’ve got a lot of people running around trying to tell people what happened, but they’re not down here to have the facts and the evidence to know what happened.”

Six lawsuits have been filed in local county court against Adair Grain, which owns and operated West Fertilizer. Those suing include people injured in the blast and insurance companies representing businesses that were damaged.

Kistner said the owners of the plant have cooperated with investigators, but declined to say what documents they had turned over.