SYDNEY (Reuters) - Stricter screening of passengers and luggage at Australian airports will stay in place indefinitely after police foiled an alleged “Islamic-inspired” plot to bring down a plane, which local media said may have involved a bomb or poisonous gas.

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The ramped-up security procedures were put in place after four men were arrested at the weekend in raids conducted across several Sydney suburbs. The men are being held without charge under special terror-related powers.

The Australian Federal Police (AFP) would not confirm media reports the alleged plot may have involved a bomb disguised in a meat grinder or the planned release of poisonous gas inside a plane.

AFP Commissioner Andrew Colvin told reporters on Monday plot specifics were being investigated.

“What you are seeing at the moment is making sure that there is extra vigilance, to make sure that we aren’t cutting any corners in our security, to make sure that we are absolutely focused on our security,” Colvin said.

Two U.S. officials familiar with the Australian arrests said the investigation was not a sting operation, but a developing plot that was detected. One of the officials said the alleged plot was “fairly well along” when Australian authorities disrupted it. The target, the other official said, appears to have been a commercial flight from Sydney to the Gulf.

Two other U.S. officials, also speaking on condition of anonymity, said communications between the plotters in Sydney and members of Islamic State in Syria were intercepted by a foreign intelligence service. The officials declined to identify the service, and both American and British officials refused to confirm or deny playing a role in detecting the alleged plot.

Australian police on Monday were still searching several Sydney properties for evidence. Pictures showed forensic-specialist officers wearing masks and plastic jumpsuits inside the properties and combing through rubbish bins outside.

Immigration and Border Protection Minister Peter Dutton told reporters in Melbourne on Monday that the alleged plot to down an aircraft could prompt longer-term airport security changes.

“The security measures at the airports will be in place for as long as we believe they need to be, so it may go on for some time yet,” said Dutton.

“It may be that we need to look at the security settings at our airports, in particular our domestic airports, for an ongoing enduring period,” he said.

Dutton advised passengers to arrive at airports three hours before international flights and two hours for domestic flights in order to clear the heightened security.

Inter-state travelers are subjected to far less scrutiny than those traveling abroad with no formal identification checks required for domestic trips.

Passengers at major Australian airports, including Sydney, experienced longer-than-usual queues during the busy Monday morning travel period. A Reuters witness said the queues had disappeared at Sydney Airport by lunch-time.

A source at a major Australian carrier said the government had instructed airlines and airports to ramp up baggage checks, with some luggage searches now being conducted as passengers queued to check in their bags.

Counter-terrorism police have conducted several raids recently, heightening tensions in a country that has had very few domestic attacks.