Lm Otero / AP

Investigators said ammonium nitrate caused the explosion at a fertilizer plant in West, Texas, on April 17. Fourteen people died and more than 200 more were injured in the blast.

Investigators have determined that ammonium nitrate was the cause of the explosion at a West, Texas, fertilizer plant last month that killed 14 people and injured some 200 more, a spokeswoman for the Texas state fire marshal's office said Tuesday.

"The investigators have been able to narrow down the origin to the fertilizer and seed building on site, and we also know that what caused the explosion was the ammonium nitrate," said Rachel Moreno, a spokeswoman for the Texas State Fire Marshal's Office. "What we don't know is exactly why."

The fire marshal's office has been leading the investigation of the April 17 blast, along with the federal Alcohol, Tobacco, Firearms and Explosives agency (ATF).

Ammonium nitrate is a dry fertilizer mixed with other fertilizers such as phosphate and applied to crops to promote growth. It can be combustible under certain conditions, and was used as an ingredient in the Oklahoma City bombing in 1995 that left 168 people dead.

Slideshow: Fertilizer plant explosion in Texas

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A huge blast rocked a small Texas town, killing 14 people and injuring some 200 more.

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Any major bomb investigation is an elaborate, high-tech piece of choreography by city, state and federal agencies with two goals: helping investigators figure out who built and planted the explosive device or devices and preserving evidence so that they can be brought to justice.

And the forensic investigation established soon after the twin blasts at Monday’s Boston Marathon has already begun yielding results. On Tuesday afternoon, law enforcement officials had indications that the bombs were made from pressure cookers filled with explosives, nails and BBs, and that they were placed in black nylon bags.

The work is far from over. Fragments, blood smears and explosive residue from the Boylston Street crime scene -- all of it will be carefully cataloged and examined. Every frame of video and every photo will be scrutinized, a mammoth undertaking in light of authorities’ pleas for spectators to turn over their images.

There’s other evidence to consider, too. A smell of sulfur in the air could indicate smokeless black powder was used. The size and color of a fireball could point to certain additives. Certain bomb mechanisms -- a type of fuse, a type of timer -- could be signatures of a particular group.

Alex Trautwig/Getty Images

A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during Boston Marathon on April 15.

"In an investigation of this nature, no detail is too small," Attorney General Eric Holder said Tuesday.

As the data piles up, investigators will begin to get some idea of who was behind the horrific act: an amateur or a professional bomb-maker, a home-grown lone wolf or a foreign-sponsored terrorist gang.

Early indications were pointing toward a sophisticated creator, as the bombs were designed and placed to act like a "homemade claymore," a powerful, directional anti-personnel device, sources involved in the investigation said.

These and other sources say that the triggering mechanism appears to have included a battery pack and a circuit board, the elements they said of a sophisticated triggering mechanism. Both of those elements were recovered at the scene.

"It appeared to be built from scratch but with a sophisticated triggering mechanism. And frankly, at the end of the day, all bombs are crude devices, and it is the way they are triggered that can be sophisticated," said one official with strong knowledge of explosives. "They functioned as designed."

In these kinds of investigations, the forensic process begins as soon as police have done what they can to preserve human life and clear the area, with bomb technicians and emergency service cops canvassing for devices that may not have exploded yet.

Protection from contamination
In Boston, officials confirmed they used controlled explosions -- usually done with water cannons -- on five suspicious packages that turned out not to be bombs.

Afterward, uniformed officers -- in Boston's case, the National Guard -- secure the perimeter of the blast site to protect the evidence from contamination until the specialists can bag, tag and transport it to a central location, where a prosecutor would ideally be supervising the chain of custody, local and federal officials say.

Boston's FBI Special Agent in Charge Richard DesLauriers said that recovery effort officially began Tuesday morning.

In these investigations, the entire area is photographed and investigators begin a grid search, working outward from the seat of the blast, swiping fragments for explosive residue and gathering anything that could be a clue. In Boston, debris has been found on roofs and embedded in buildings. What’s recovered will be sent to the FBI lab in Quantico, where it will eventually be logged on a grid, according to law-enforcement officials.

The painstaking work can have big payoffs.

If authorities can identify a type of explosive, they can try to trace where it might have been purchased. 

As House Homeland Security Chairman Michael McCaul, R-Texas, noted, if the probe reveals the bomb was set off by a cellphone call, investigators can track all the calls made at exactly that moment.

Private and city surveillance cameras can show the color and density of the smoke plume -- details that can point to bomb size and ingredients.

Damage can tell a tale
To the trained eye, damage to the area and to the victims also yield important information. In Boston, pockmarks on the buildings pointed to a bomb packed with BBs. The gruesome injuries -- legs torn from bodies -- would indicate the bomb was close to the ground, experts say. The absence of widespread ear and lung injuries is associated with a low-explosive device.

Blood from the scene is also collected. The lab can later compare the samples to the victims' types to determine if there's a swab that has no match and could belong to a suspect.

If investigators are lucky, within 12 hours they will have enough fragments to pinpoint certain aspects of the bomb -- as has happened in the Boston case. The FBI then begins building a facsimile of the device.

Boston bombing investigators were working to identify the type of timer -- whether it was a cellphone alarm, for instance -- and verify other components and substances used.

The manpower required for such tasks is massive. A probe like the one in Boston could easily involve more than 100 people in forensic collection and analysis.

In Boston, the FBI has taken the lead. The Bureau of Alcohol, Tobacco and Firearms announced it was sending 30 specialists to the city. There will be experts in dental work, DNA, blood-pattern analysis and digital video analysis working the case. Local and out-of-town agencies devoted bomb squad and forensic personnel to the investigation.

Typically, 36 hours after a bombing the forensic teams will have collected and mapped what they can and will consider releasing the scene. The hope is that by then they will also have someone in custody or some idea of who it is they're hunting.

Richard Esposito is the author of “Bomb Squad: A Year Inside the Nation’s Most Exclusive Police Unit”.

Slideshow: Boston Marathon explosions

Charles Krupa / AP

See images from the scene of the explosions.

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This story was originally published on Wed Apr 17, 2013 3:39 AM EDT