A 25-minute cellphone video left behind by the bomber whose deadly explosives terrorized Austin for weeks details the differences among the weapons he built and amounts to a confession, police said. But his motive remains a mystery.

Mark Anthony Conditt, an unemployed college dropout who bought bomb-making materials at Home Depot, recorded the video hours before he died after detonating one of his own devices as SWAT teams closed in. It seemed to indicate the 23-year-old knew he was about to be caught, said Austin police Chief Brian Manley.

Austin Community College released this 2010 ID photo of Conditt, who attended classes there between 2010 and 2012, according to the school. (Austin Community College/Associated Press)

"It is the outcry of a very challenged young man talking about challenges in his own life," Manley said of the recording, which authorities declined to release amid the ongoing investigation.

Conditt was tracked down using store surveillance video, cellphone signals and witness accounts of a customer shipping packages in a disguise that included a blond wig and gloves. Police finally found him early Wednesday at a hotel in a suburb north of Austin.

Bombs killed 2, wounded 4

Officers prepared to move in for an arrest. When the suspect's sport utility vehicle began to drive away, they followed. Conditt ran into a ditch on the side of the road, and SWAT officers approached, banging on his window.

Within seconds, the suspect had detonated a bomb inside his vehicle, blasting the officers backward, Manley said. One officer then fired his weapon at Conditt, the chief said. The medical examiner has not finalized the cause of death, but the bomb caused "significant" injuries, he said.

Law enforcement officials did not immediately say whether Conditt acted alone in the five bombings in the Texas capital and suburban San Antonio that killed two people and badly wounded four others. Fred Milanowski of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives said investigators were confident that "the same person built each one of these devices."

Investigators released few details about Conditt, except his age and that he was white. Neighbours say he was home-schooled. He later attended Austin Community College from 2010 to 2012, according to a college spokesperson, but he did not graduate.

'A weird house'

In a 2012 online blog that the college spokesperson said Conditt created as part of a U.S. government class project, he gives his opinion on several issues, often in response to someone else's commentary. Conditt wrote that gay marriage should be illegal, argued in favour of the death penalty and gave his thoughts on "why we might want to consider" eliminating sex offender registries.

In the "about me" section of the blog, Conditt wrote that he wasn't "that politically inclined" but did view himself as conservative.

Austin Police Chief Brian Manley, centre, stands with other members of law enforcement as he briefs the media early Wednesday in the suburb of Round Rock. (Eric Gay/Associated Press)

Jay Schulze, who lives in Pflugerville, said he was jogging Tuesday night when he was stopped by police and asked about the bombings. He said police flew drones over Conditt's home for about six hours between Tuesday evening and early Wednesday morning.

Schulze described the home as "a weird house with a lot of people coming and going" and a bit rundown.

'Fatal mistake'

A neighbour who watched Conditt grow up said he always seemed smart and polite. Jeff Reeb said he has lived next to Conditt's parents for about 17 years and described them as good neighbours. Conditt had visited his parents regularly, he said.

Conditt's family released a statement saying they had "no idea of the darkness that Mark must have been in." His uncle, Mike Courtney, said his nephew was a "computer geek" who was intelligent and kind.

Police warn public there may be more bombs out there 1:51

Austin was hit with four bombings starting on March 2. The first explosions were from packages left on doorsteps. Then a bomb with a tripwire was placed near a public trail. A fifth parcel bomb detonated early Tuesday at a FedEx distribution centre near San Antonio.

Representative Michael McCaul, a Republican from Austin, said Conditt's "fatal mistake" was walking into a FedEx store to mail a package because that allowed authorities to obtain surveillance video that showed him and his vehicle, along with his license plate number. From there, investigators could identify the suspect and eventually track him using his cellphone.

More bombs possible

Police warned of the possibility that more bombs had yet to be found.

"We don't know where this suspect has spent his last 24 hours, and therefore we still need to remain vigilant to ensure that no other packages or devices have been left to the community," Manley said.

Officials investigate the scene where Conditt blew himself up as authorities closed in, on Wednesday in Round Rock. (Eric Gay/Associated Press)

By late afternoon, federal officials had a "reasonable level of certainty" that there were no more package bombs "out in the public," said Milanowski, the agent in charge of the Houston division of the ATF. But authorities urged continued awareness just in case.

"We think we're on top of this, but we just don't know," FBI agent Chris Combs said.

Homemade explosives recovered

Homemade explosives were recovered from Conditt's home in Pflugerville, a community where portions of Friday Night Lights were filmed. His two roommates were detained for questioning. One was later released.

Investigators said one room in the home contained bomb components and explosive materials but no finished bombs. They were analyzing Conditt's internet history to find out how he learned to make bombs.

Isaac Figueroa said he and his brother heard sirens and helicopters around 2 a.m. Wednesday in the area and drove toward them, then cut through nearby woods on foot after they hit a police roadblock.

AUSTIN BOMBING SUSPECT IS DEAD. Great job by law enforcement and all concerned! —@realDonaldTrump

The 26-year-old said they saw an SUV that was pinned between large vans and "looked like it had been rammed off the road." He said police later deployed a robot to go examine the vehicle.

The suspect's death followed a day of rapid-fire developments in the case. On Tuesday, the bomb at the FedEx shipping centre in suburban San Antonio exploded on a conveyer belt.

Later, police sent a bomb squad to a FedEx facility outside the Austin airport to check on a suspicious package. Authorities subsequently said that package contained an explosive that was tied to the other bombings.

Officers then recovered footage of Conditt wearing a blond wig and gloves as he turned over packages to send at a FedEx store in south Austin. That was enough to set in motion the manhunt that ended with Wednesday's fatal explosion.