Austin bombing suspect used a child safety sign to conceal one of his explosives

Investigators still do not know the motive behind a deadly serial bombing spree that terrorized residents in Austin, Texas, but new documents indicate the suspect was so calculating and callous he used a "Drive Like Your Kids Live Here" sign to conceal one of his explosives.

Mark Anthony Conditt, 23, blew himself up with one of his homemade, shrapnel-loaded bombs on March 20 as a SWAT team moved in to arrest him.

"The investigation continues. We're looking at his intent, his motivations," John Bash, the U.S. attorney for the Western District of Texas, said at a news conference Monday evening. "There are no further suspects at this time and we do not have any reason to believe there's a further destructive device out there."

A federal judge in Texas unsealed on Monday an eight-page affidavit prosecutors filed in support of an arrest warrant for Conditt, an unemployed college dropout, just hours before he killed himself in his vehicle in an Austin suburb.

The heavily redacted affidavit shows that Conditt built seven explosive devices, using many of the same parts he purchased at hardware stores.

The bombing spree began on March 2 when Anthony Stephan House, 39, was killed opening a package bomb left on his porch in Austin. Ten days later, on March 12, a package bomb killed 17-year-old Draylen Mason and critically injured his mother inside their Austin home. That same day a 75-year-old woman was critically injured by a package bomb left at her home.

A day after the two March 12 bombings, a man believed to be Conditt walked into a Home Depot in Round Rock, Texas, and purchased several signs, including one reading, "Drive Like Your Kids Live Here."

On March 18, a resident in the Travis Country neighborhood of Austin spotted the same sign with red backing and white letters erected in a yard, where later that day an explosive triggered by a tripwire went off and injured two men.

"I believe, based on interviews with the witness and the preliminary analysis of the scene, that the sign was utilized to facilitate the concealment of the explosive device," Reynaldo Alatorre Jr., an agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, wrote in the court affidavit.

Another package bomb exploded on March 20 at a FedEx processing center in Schertz, Texas, that injured a worker. Later that day, investigators discovered a suspicious package at a FedEx facility near Austin–Bergstrom International Airport in Austin.

"That package was X-Rayed and an explosive device was found inside," Alatorre wrote in the affidavit. "Law enforcement was able to render the device safe. Preliminary analysis of the device revealed it consisted of a PVC pipe casing with a metal pipe inside surrounded by shrapnel."

Investigators determined that a trigger used on the bomb was designed to explode "when a flap of the package was opened."

The bomb was dropped off at the FedEx facility by a man matching Conditt's description. Security video showed him "wearing gloves and a hat in the store," according to the affidavit.

"He paid in cash. It appeared that he was wearing a wig," Alatorre wrote in the document.

Of the bombs used to kill and maim victims in the rampage, "all six explosive devices used shrapnel," according to the document.

Investigators also discovered evidence that Conditt had gone on a buying spree for bomb-making components on Feb. 27 when he used a credit card issued in his name to purchase five battery holders with snap connectors at a Frye's electronics store in Austin, according to the affidavit.

"Preliminary analysis of the explosive devices revealed that all six explosive devices utilized a ... battery holder with snap connector," the affidavit reads.

The probe also uncovered "voluminous computer records" from Conditt's home that investigators are still combing through, Bash said at Monday's news conference.

Conditt also left behind a 25-minute recording on his phone confessing to making seven bombs, including the one he used to kill himself, officials said.

While investigators continue to try to learn the rationale behind the explosions, the recording did not reference terrorism or hate, Austin Police Chief Brian Manley said during a March 21 news conference. Manley has since described Conditt as a "domestic terrorist."

Conditt's recorded confession has not been made public, but Manley described it as "the outcry of a very challenged young man talking about challenges in his personal life."