PFLUGERVILLE, Texas >> Investigators combed through mounds of evidence on Thursday, trying to piece together a motive for a bombing spree that killed two people and terrorized the Texas capital.

Included in the evidence is a 25-minute recording made by the suspect, Mark Conditt, 23, the day 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 today. “No other people, no other evidence, any other clues like that.”

ATF agents remained at Conditt’s home today in the normally tranquil neighborhood of Pflugerville, just north of Austin. An ATF vehicle was parked on the street near Conditt’s house, which was blocked off, though the official presence had thinned considerably. Besides the discovery of the recording on Conditt’s cellphone, the authorities said they found “componentry and the homemade explosive material” in his house.

Still, investigators, as well as friends and family, were unable to answer the questions on everyone’s minds: How had Conditt learned to make the bombs, and, more important, why had he done so?

“I would love to understand what motivated him to kill,” said Sierra Davis, a longtime friend who recently lost touch with Conditt.

The attacks began this month when a package bomb exploded on an Austin front porch, killing Anthony Stephan House, 39, as he helped his daughter get ready for school. Ten days later, two more bombs exploded outside homes, one of which killed Draylen Mason, 17, a promising classical musician. In all, four people were injured in five explosions.

The bombing spree came to a fiery end Wednesday morning when, the police said, Conditt ignited another explosive device inside a vehicle he was driving as officers chased him along a stretch of Interstate 35 in Round Rock.

On Wednesday, the Austin police chief, Brian Manley, said a full understanding of Conditt’s motive was elusive, even though he had listened to the recording.

He called the recording a “confession” and said Conditt had spoken extensively about the similarities and differences between the bombs he had built.

Investigators said that the crucial first break in the case came when Conditt mailed the packages at the FedEx store this week. Surveillance tape in and near the FedEx store led to a pickup that investigators began tracking.

A local television affiliate, KXAN, reported that ATF agents also had access to security video from an electronics store in North Austin. The station reported that ATF agents had visited a Fry’s Electronics store more than a week ago, looking for receipts and surveillance video of people who had bought components found in the bombs.

Jen Meyer, a manager at the store, told KXAN that Conditt had visited the store on Feb. 27, days before the first bomb exploded.

“He was purchasing some battery holders,” Meyer said. “He looked like hundreds of customers we have every day.”

In Conditt’s neighborhood today, residents were trying to return to normal after law enforcement officials surged in Wednesday and ordered an evacuation while they searched for explosives in Conditt’s house.

“It’s good to see a little quiet in the neighborhood,” said Gloria Kuempel, 79, whose country-style home on Wilbarger Street is a half-block from the police barricades at the entrance of Conditt’s street.

“I often thought about going over to welcome him to the neighborhood, but I never did,” she said, adding that she never suspected that the young man she saw working outdoors to remodel his house would turn out to be a serial bomber.

Speaking at a City Council meeting in Austin, Mayor Steve Adler said the city had been through a harrowing few weeks.

“There was a feeling that there was not much that we could do, there was a collective helplessness,” Adler said. He suggested that people introduce themselves to their neighbors: “You’re less afraid of the people around you when you know who they are and you can notice things that are out of the ordinary.”