CHICAGO (AFP) - Police in the US state of Texas expressed concern on Monday (April 9) that releasing a confession tape made by the suspect in a series of bombings in Austin could "inspire" other would-be killers.

Over three weeks in March, six package bombs were left at people's doorsteps in the state capital, concealed along a walking path, or sent via FedEx, in seemingly random attacks.

Two people were killed, including a 17-year-old boy, and several others were injured when they picked up or opened the packages, or stumbled over a tripwire.

The suspected bomber, Mark Conditt, recorded a confession on his mobile phone prior to killing himself as police closed in.

The contents of that recording have not been made public, and authorities revealed that they were debating whether to do so after the investigation concludes - likely in another several months.

"From the review of past criminals, we are very concerned, and the last thing we want is this to inspire somebody else," said FBI special agent Christopher Combs, who has been in charge of the federal agency's response.

"The subject in the audio confession says a number of statements that concern us, and we just don't want that to live forever on the Internet," special agent Combs said, without offering further details.

His comments came after a judge released a redacted legal filing, offering new information about the case.

Investigators have determined that the explosive devices were made with plastic and metal piping and contained shrapnel, according to the document.

They have also determined the triggering mechanism, but that information was redacted.

At the news conference, authorities offered no information about a possible motive or how the victims were targeted.

They also revealed that they have identified no other suspects, despite briefly detaining and interviewing Conditt's two roommates.

US Attorney John Bash said investigators were looking through Conditt's "voluminous computer records" for additional clues.

"The investigation continues. We're looking at his intent, his motivations," Mr Bash told the news conference.

The Austin American-Statesman reported in March that Conditt described himself on the confession recording as a "psychopath" who had been disturbed since childhood.

"I wish I were sorry, but I am not," Conditt said, according to the newspaper.