The security alert that closed two shopping centres and Australia Post's airport sorting centre and involved the electorate offices of the Treasurer and Premier has ended.
Police converged on the Premier's Cottesloe electorate office earlier this evening after a suspicious package was identified.
Up to five suspect envelopes - three at the sorting centre, one at Treasurer Mike Nahan's electorate office and the item at the Premier's office - were at the centre of the alert.
Two workers at the Premier's electorate office were told they could not leave until it was safe.
But a short time ago, Forensic Division Superintendent Tony Flack said the envelopes contained a written threat, but no harmful substances.
He said the Premier and Treasurer's personal security arrangements would be increased.
"The threat clearly indicated that something more serious might happen, but didn't indicate the reasons why," Supt Flack said.
"The threats were not directed at the individuals concerned, and they were not about a particular policy."
Members of the bomb squad searched the Stirling Highway office with a bomb detection dog, while analysis of the envelope and other evidence began in a mobile forensics lab.
Supt Flack said further forensic examination and witness interviews would take place tomorrow.
"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."
He said members of the State Security Unit were being advised as further information about the threats came to light.
"The Premier has not been interviewed," he said.
"Our state security unit will be putting increased measures in place."
He said the rest of State Cabinet had been put on alert for similar suspicious letters or other threats.
The Riverton Forum Shopping Centre, where Dr Hahan's electorate office is located, was closed earlier this afternoon after the suspicious package was found.
Police says three envelopes, one of them addressed to The West Australian, at the sorting centre have been analysed and contain no dangerous substances.
But an envelope addressed to Dr Nahan is still being analysed.
Officers with a HAZMAT unit have left but police remain at the scene.
A threat on one of the letters addressed to The West Australian said "if the powder doesn't get you the bomb will".
It is understood the envelope reached the mail room in Dr Nahan's office.
It is unclear if it was opened.
It was discovered this afternoon and sparked the police response.
At least three of the envelopes contain a "powder-like substance".
Police said it was sheer luck that the envelopes were detected at the Australia Post sorting centre.
One envelope was disturbed by an automatic mail sorting machine last night and was put aside.
The alert was raised this morning when the item was examined, police said.
Road blocks have been set up and emergency services are at the scene at Boud Avenue.
Police said the three items were deemed suspicious because of words written on them.
The Tactical Response Group bomb unit is assessing the items.
Seven News reporter Rebecca O'Donovan says a worker has been told a suspicious package found at Australia Post was smouldering. Roads.
In a statement, WA Police said there were several road closures in place that will impact on Dunreath Drive and Brearley Avenue.
Brearley Avenue is open all the way to the Domestic Airport, but there may be delays due to the build up of traffic.
Dunreath Drive is closed.
People heading to Perth Airport are advised to allow extra time because of the expected build up of traffic.