PFLUGERVILLE, Texas—Investigators on Thursday finished questioning the second roommate of the man they say spent weeks planting bombs around Texas’ capital before using one of his own devices to blow himself up, but a more complete picture of the 23-year-old community college dropout remains elusive.

Police said they’d released the second of two people who shared a home in Pflugerville, just north of Austin, with serial bombing suspect Mark Conditt. The other roommate was arrested Wednesday and released hours later. Investigators haven’t named them, saying they weren’t under arrest.

Conditt made a 25-minute cellphone recording before his death. It was recovered Wednesday, after he detonated one of his own bombs along the side of Interstate 35 just outside of Austin as a SWAT team moved in.

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But investigators say the recording provides few clues as to Conditt’s motives — and they’ve refused to release it publicly, citing the ongoing investigation.

Conditt built bombs planted in different parts of the city that killed two people and severely wounded four others over three weeks starting on March 2. He began by placing explosives in packages left overnight on doorsteps, then rigged an explosive to a tripwire along a public trail. Finally, he sent two parcels with bombs via FedEx.

All of the early victims were Black or Hispanic, though the tripwire bomb injured two white men.

As fear grew, Conditt eventually was tracked down through store surveillance video, cellphone signals and witness accounts of a customer shipping FedEx packages in a disguise that included a blond wig and gloves.

Police found him early Wednesday at a hotel. And when his car moved, they followed and forced him off the road, setting up the confrontation that ended in an explosion.

Austin Mayor Steve Adler praised police officers for stopping Conditt’s sport utility vehicle before he could get onto the highway, even though they were aware he likely had explosive devices inside and they did not have as much backup as planned.

“We hear about folks who run into danger to keep us safe and we had one of those moments in very graphic and specific detail,” Adler said at a news conference Thursday.

NAACP Austin President Nelson Linder said the manhunt’s end was a relief but that the public needs answers on what motivated Conditt, including whether the first two victims were targeted because they are from prominent black families.

“I don’t think it’s random at this point,” Linder said. “We’re going to withhold our judgment and keep searching for information and why he killed those people.”

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Investigators have released few details about Conditt, who was unemployed, home-schooled and attended Austin Community College until 2012 but didn’t graduate.

The Austin American-Statesman reported Thursday that a bomb Conditt made and shipped via FedEx had been addressed to an employee at a downtown Austin spa. It was intercepted at a processing centre without exploding.

Austin, Texas police chief Brian Manley says the suspect in a string of bombings in the city is dead, and that the suspect blew himself up in his vehicle on Wednesday morning as officers approached. (The Associated Press)

Anita Ward works at Austin Med Spa and says that police and federal agents told her that her daughter, who also works there, was meant to be the recipient of the unexploded package.

Ward declined to name her daughter, who she said doesn’t know Conditt. Police haven’t commented.