Investigators may find the cause of the fire and subsequent explosion in West by May 10, the state fire marshal told lawmakers Wednesday.

The West Fertilizer Co. plant explosion killed 15 people, injured more than 200 and destroyed 142 homes and several buildings, including a nursing home and two schools.

Speaking to the House Homeland Security and Public Safety Committee, State Fire Marshal Chris Connealy likened the ongoing investigation to an archaeological dig and said investigators — who represent 28 state and federal agencies — are working to reconstruct the cause of the explosion by examining the 14.9 acres affected by the blast.

“This is a very complex event as you can imagine, and we want to make sure we do it correctly,” Connealy said.

The marshal said investigators have ruled out a natural event, such as a lightning strike, as the cause, but three other categories of probable causes — accidental, incendiary and “undetermined” — are still on the table.

Connealy spoke at the first hearing regarding the explosion, where representatives from eight state agencies assessed their role in overseeing and regulating plants such as the one in West.

As of February, the plant contained 270 tons of ammonium nitrate, a chemical with explosive capabilities, said David Lakey, commissioner of the Texas Department of State Health Services. He said 41 other facilities in the state also have large amounts of the chemical.

Lakey said facilities with fewer than 10,000 pounds of ammonium do not have to report possession of the chemical because it is not included on the U.S. Environmental Protection Agency’s list of dangerous substances.

Officials said local fire departments are authorized to inspect these facilities. Texas Emergency Management Chief Nim Kidd said he had not directed fire departments in cities with similar plants to conduct inspections but said he could do so upon lawmakers’ request.

State Rep. Joe Pickett, D-El Paso, chairs the House committee and said the hearing was intended to clarify agencies’ role in handling the aftermath of the explosion and overseeing similar plants. He said he did not intend for lawmakers to get involved in the investigation or “to point fingers.”

“I want to take it at a pace that is not a knee-jerk reaction to things,” Pickett said after the hearing.

Legislators in Washington will also examine the cause of the blast.

On Tuesday, U.S. Sen. Barbara Boxer, D-Calif., who chairs the Environment and Public Works Committee, said the committee would investigate the circumstances surrounding the explosion. Boxer sent letters to the Chemical Safety Board and Environmental Protection Agency asking how they would follow up on the situation.

“I cannot rest until we get to the bottom of what caused the disaster in West, Texas, and the tragic loss of life,” Boxer said in a press release. “It is critical that we find out how this happened.”