SYDNEY (Reuters) - Australian police said on Tuesday they had released one of four men arrested in raids last weekend that foiled an “Islamic-inspired” plot to bring down a plane.

Local media said the plot may have involved a bomb or poisonous gas.

A 50-year-old man was released on Tuesday night, the Australian Federal Police (AFP) said in a statement, adding that no criminal charges had been filed against him. The other men remain detained without charge, the statement said.

The AFP has not released details of the plot, although a U.S. official familiar with the arrests told Reuters that the target appeared to have been a commercial flight from Sydney to the Persian Gulf.

Abu Dhabi’s Etihad Airways said on Tuesday it was assisting the AFP in the investigation.

Australia has been on heightened alert since 2014 for attacks by home-grown militants returning from fighting in the Middle East, or their supporters, but has suffered few domestic attacks.

The 2014 Lindt cafe siege in Sydney, in which the hostage-taker and two people were killed, was Australia’s most deadly violence inspired by Islamic State militants.