Police in Australia are continuing to hunt for more evidence as they investigate a major terror plot targeting the aviation industry.

Four people – two Lebanese-Australians men and their two sons – were arrested in raids in Sydney on Saturday, following an intelligence tip-off. Australian Prime Minister Malcolm Turnbull described the plot as “Islamic-inspired terrorism.”

“We have succeeded in disrupting a major terrorist plot to bring down an airplane,” Turnbull said. “Good intelligence, great police work, great investigation, great co-ordination has enabled us to disrupt that plot.”

AUSTRALIAN POLICE ARREST FOUR IN ALLEGED PLOT TO BOMB AIRCRAFT

Police are conducting forensic work at five properties linked to the suspects, but refused to elaborate on details of the planned attack. Australian Federal Police Commissioner Andrew Colvin would not confirm reports in the Australian media that the plot involved poisoning passengers with a toxic gas, or using a homemade bomb smuggled onto a flight in hand luggage.

“The plot that we are investigating, we believe was an attempt to put a device onto an aircraft. But, beyond that, the speculation is just that – it’s speculation,” Colvin said.

AUSTRALIA AIRPORT SECURITYS STAY HIGHTENED OVER TERROR PLOT

Colvin also refused to comment on unconfirmed reports that the suspects were not previously known to police.

Since the arrests, security been stepped up at both domestic and international terminals of Australia’s major airports. Authorities warn the new measures may be in place indefinitely.

“We need to remind the public the threat is there, it’s why we’ve ramped up the security at our domestic and international airports,” said Peter Dutton, Australia’s Immigration and Border Protection Minister. He advised passengers to arrive early at airports to allow time for additional screening.

Australian authorities have disrupted 13 terror threats over the past few years – but nothing on this scale. Turnbull said the terror threat level would remain the same, at “probable,” despite these latest developments.

The four suspects have not yet been charged. Under Australian counterterrorism laws, they can be held for seven days for questioning.