The Lebanese men were reportedly building an incredibly sophisticated, “non-traditional” improvised explosive device that emits a poisonous, sulfur-based gas, The Australian reports.

Police conducted a raid Saturday in Sydney to arrest the men, all related by blood and marriage, when they received a tip from a foreign intelligence service and discovered the device was nearing completion. The arrested men are dual nationals and had close contact with ISIS in Syria.

According to The Australian, this contact with ISIS was absolutely crucial in moving the cell’s plans forward, as apparently members of the group did not have the ability to build or deploy the device on their own. It’s unclear how the group was going to attempt to smuggle the device on the plane headed to the Middle East, but the reason police officers decided to move to arrest them is because the group was making alarming progress.

“At this time, we don’t have a great deal of information on the specific attack, the location, date or time,” Australian Federal Police Commissioner Andrew Colvin said. “However, we are investigating information indicating that the aviation industry was potentially a target of that attack.”

The police made the arrests on the authority of legislation that permits detention for seven days without charge.

Unlike usual foiled terror plots, the men were unknown to police prior to the arrests over the weekend. In fact, they had remarkably clean records, with the worst infraction being a negligent driving offense against one of the four.

Authorities have ramped up airport security, following the arrests.

“Every day, every hour, we are focused on ensuring that our defenses against terrorism are stronger than ever,” Australian Prime Minister Malcolm Turnbull said.

“That our co-operation is tighter than ever,” Turnbull added. “That our co-ordination is swifter than ever before. Now, we have strong transport, security systems in place in Australia, to prevent acts of terrorism.”