Envelopes containing white powder and a threat "if the powder doesn't get you, the bomb will" were among five suspicious packages that forced Colin Barnett's staff to be quarantined and left the offices of Treasurer Mike Nahan and _The West Australian _on alert last night.
Detectives from the serious and organised crime division are investigating who sent the letters.
The first was found by chance after a sorting machine tore the envelope at the mail sorting centre at Perth Airport.
At least three of the suspicious letters, all believed to be plain white business envelopes, contained a similar substance and typed threat, including one addressed to _The West Australian _.
Senior management at the newspaper were briefed by two police officers and told to be on alert for any suspicious mail arriving in the coming days.
At least two female staff at the Premier's Cottesloe electorate office spent several hours quarantined in the building as they waited for officers from the bomb squad and scientists from the ChemCentre to analyse the envelope and learn if they had been exposed to toxins.
Staff at Dr Nahan's Riverton office were swabbed after one opened a letter that contained a powder and a typed threat. Police also closed nearby businesses and sniffer dogs were used at Dr Nahan's office.
It is believed another of the suspicious letters intercepted at the mail centre was addressed to the Taxi Control Board.
Tests later showed the substances in the suspicious envelopes were not hazardous.
It is believed none of the letters' intended recipients had been the target of recent threats.
Neither Mr Barnett nor Dr Nahan were at their offices when the security scares unfolded.
But their personal security arrangements have been stepped up. Other Cabinet members have also been put on alert.
Forensic's Supt Tony Flack said the threats were not directed at individuals and were not about a particular policy.
"All the envelopes share similar characteristics, which gives us the indication it was sent by the same author," he said. "The message was the same in all the envelopes."
The massive emergency operation was sparked when staff at the Perth Airport mail centre noticed powder spilling from a torn envelope addressed to _The West Australian _and raised the alarm.
Acting Insp. Ash Goy said if it had not been opened, it would have "had free passage" to the newspaper's Osborne Park office.
"The potential for any substance to get through the mailing system is alarming for police," he said. "On this occasion it would have been luck that it was intercepted before it got there."
Envelopes containing white powder have been at the centre of numerous international scares.
In the US after the September 11 terror attacks, anthrax-laced letters were mailed to politicians on Capitol Hill and media outlets in New York and Florida.
Five people were killed, including postal workers, and 17 others became sick.