Trail to Austin bombing suspect combined high-tech and old-fashioned techniques

Three weeks of terror in central Texas ended before dawn Wednesday in a ditch at the side of an interstate outside Austin, with an explosion that killed a young man suspected in a string of deadly bombings across the state capital.

That final blast, triggered as police closed in on Mark Anthony Conditt, 23, marked the culmination of several coordinated investigations conducted by hundreds of federal and local authorities, some high-tech and some old-fashioned.

They worked in plain view and in seclusion, assembling pieces of a case that expanded to include five explosions since March 2 — three at homes that killed two and injured two, one triggered by a tripwire that injured two, and one in a package at a FedEx distribution center near San Antonio on Tuesday.

First, there were examinations of the bombs themselves by teams from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, who collected, reassembled and studied the pieces of devices that detonated. In this phase, experts say, just about anything is a possible clue: the types of explosives, detonators and wires; surfaces containing potential DNA or fingerprints; labels, tape and stickers; tiny tool marks. The assembly itself might reveal unique identifiers.

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In this case, many of the bombs’ components — galvanized steel pipe, a form of a low-level explosive, different types of shrapnel — were fairly rudimentary, available at home goods and electronic stores, multiple law enforcement officials told NBC News. Indeed, Rep. Mike McCaul, R-Texas, told NBC Austin affiliate KXAN that Conditt purchased some of the materials at a local Home Depot. The mechanics of the bombs were also of low sophistication, officials said.

But investigators noticed something peculiar about the batteries used.

Amid the remnants of the five explosions, investigators found “exotic” Asia-made batteries that had been ordered online, multiple senior law enforcement officials briefed on the investigation told NBC News.

This was a sign that a single person was responsible, and allowed investigators to link the blasts.

Another break came from closely guarded software that allows FBI agents to use cell tower signals to locate and track individual cellphones.