FILE - In this March 19, 2018, file photo, federal investigators work near the site of an explosion in Austin, Texas. Federal and local officials have scheduled a news conference Monday, April 9, 2018, for an update into the investigation of a series of package bombs in Austin that killed two people and seriously wounded four in March before suspect Mark Conditt blew himself up as police closed in. (AP Photo/Eric Gay, File)

AUSTIN, Texas - Federal officials said Monday that the suspect in a series of fatal Austin package bombings used PVC pipe casing, a metal pipe and shrapnel in his attacks that killed two people and seriously injured four others.

Authorities released an affidavit used to support an arrest warrant for suspect Mark Conditt. Investigators used bomb evidence, video surveillance, interviews and phone records to chart a path to Conditt. Police say he blew himself up March 21 as officers closed in to make an arrest.

The document still redacts details about some of the explosive materials used, confidential witnesses who were interviewed and phone numbers.

John Bash, U.S. attorney for the Western District of Texas, said authorities also dismissed an arrest warrant that had been issued hours before police say Conditt killed himself.

Bash said the investigation continues although no other suspects have been arrested "and we no reason to believe there are other suspects." Authorities have yet to determine a motive for Conditt's attacks and the affidavit gave no hint of one.

"We are looking through very voluminous computer records to examine his intent, his motivations," Bash said.

Beginning March 2, police say Conditt, 23, planted bombs in different parts of Austin. He began by placing explosives in packages left overnight on doorsteps, killing 39-year-old father Anthony Stephan House and 17-year-old musician Draylen Mason and critically injuring 75-year-old Esperanza Herrera. He then rigged an explosive to a tripwire along a public trail, injuring two young men who crossed it. Finally, he sent two parcels with bombs via FedEx, one of which exploded and injured a worker at a distribution centre near San Antonio.

Investigators discovered a roughly 25-minute recording that Conditt had made on a cellphone allegedly confessing to the crimes. Authorities have not released that recording.

Christopher Combs, special agent in charge of the FBI's San Antonio office, said authorities worry releasing the recording could inspire copycat bombers.

"When you look at past active shooters or mass murderers, what we find is they study the previous events," Combs said. "We are concerned it could inspire other people to do other acts. (Conditt) says a number of statements that concern us and we just don't want that to live forever on the internet."

News from © The Associated Press, 2018