Police believe they know where suspicious envelopes that sparked recent security scares were posted but say further forensic tests will be carried out after preliminary analysis failed to uncover DNA or fingerprints to identify the perpetrator.
Investigators will search for security vision from the area where the packages containing white powder and threats were posted to try to track down who sent the mail, while forensic experts will examine adhesive address labels, postage stamps and sticky tape from the plain white envelopes.
Acting commander for State crime, Tony Flack, said police believed the mail was all sent from the same place in the metropolitan area, likely on the same day. But he would not say where that was.
A massive security operation was launched on April 16 after two plain white envelopes containing white powder and a typed threat, one addressed to The West Australian, were intercepted at a mail processing centre at Perth airport.
Packages were also delivered to the electorate offices of Premier Colin Barnett, Treasurer Mike Nahan and Parliament House, sending them into lockdown until the powder was analysed.
The substance was flour but police are taking the threats seriously, with messages in the envelopes warning that if the powder did not kill, the bombs would.
Tests are still being done on the contents of a similar package that arrived at Troy Buswell's Busselton electorate office on Tuesday.
Acting Cdr Flack said preliminary analysis on the first envelopes and typed threats had not retrieved any DNA or fingerprint evidence.
More testing would be done on the sticky labels and stamps to see if DNA or fingerprint impressions could be recovered.
Acting Cdr Flack said it had been difficult to identify potential offenders because there was no clear motivation for the threats.