PFLUGERVILLE, Tex. — Investigators sifted through mounds of evidence and hundreds of leads on Thursday, hoping to unearth a motive for a bombing spree that killed two people and terrorized the Texas capital.

Included in the evidence is a roughly 25-minute recording made by the bombing suspect, Mark A. Conditt, 23, the night before he died in an explosion as law enforcement officials closed in.

“We’ve got to follow up on everything, just to make sure there’s nothing that we missed,” Nicole Strong, a special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, said on Thursday. “No other people, no other evidence, any other clues like that.”

Among the questions that investigators, as well as friends and family, were trying to answer: How had Mr. Conditt learned to make the bombs, and, more important, why had he done so?