Security has been increased at Sydney Airport since Thursday.

:: Security remained heightened in airports around Australia with more intense screening of luggage after law enforcement officials thwarted what a police chief described on Monday as a “credible attempt to attack an aircraft.”

Prime Minister Malcolm Turnbull and Border Protection Minister Peter Dutton declined to comment on newspaper reports that Islamist extremists planned to kill the occupants of a plane with poison gas and that a homemade bomb was to be disguised as a kitchen mincer.

“Police will allege they had the intent and were developing the capability,” Turnbull told Australian Broadcasting Corp. Turnbull announced on Sunday that “a terrorist plot to bring down an airplane” had been disrupted, but revealed few details.

Four men arrested in raids in Sydney late Saturday — two Lebanese-Australian fathers and their sons — had yet to be charged. Australian Federal Police Commissioner Andrew Colvin said a court ruled Monday that the four could be detained without charge for seven days from their arrest under counterterrorism laws.

“We believe we have disrupted a legitimate and credible attempt to attack an aircraft,” Colvin told reporters without elaborating.

“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 — speculation,” he said, adding that police had “many working theories.”

Colvin and the government will not comment on media reports that the suspects were not previously known to Australian security officials and that their arrests followed a tip from a foreign intelligence agency.

Australian newspapers cited multiple anonymous sources saying that the plotters were constructing a “non-traditional” explosive device that could have emitted a toxic, sulfur-based gas to kill or immobilize everyone on the aircraft.

Sydney’s The Daily Telegraph newspaper reported that the plotters planned to make a bomb from wood shavings and explosive material inside a piece of kitchen equipment such as a mincing machine.

Police raided five homes Saturday and removed a domestic grinder and a mincer used to make sausage, the newspaper said.

The plot involved smuggling the device onto a flight from Sydney to the Middle East, as carry-on luggage, the newspaper said.

Fairfax Media reported that the bomb was found in a home in the inner-city suburb of Surry Hills, a few doors from the local mosque.

Security has been increased at Sydney Airport since Thursday because of the plot and has since been increased in all major Australian international and domestic terminals.