One man from four who were arrested over the weekend over an alleged terrorist plot to bomb a plane traveling from Australia has been release, police announced on Wednesday.

The 50-year-old, named by his lawyer as Adbul Merhi, was arrested after five homes were raided in the city of Sydney on Saturday, sparking a tightening of security at major domestic and international airports.

“One of those men was released from police custody last night (Tuesday) without being charged with a criminal offense,” police said in a statement.

“This investigation remains ongoing, and further information will be provided at an appropriate time.”

Merhi’s lawyer Moustafa Kheir said he would review how police acted towards his client.

“My client Abdul Merhi has been released without charge,” he wrote on Twitter.

“Tough few days, but he’s relieved the truth is out. I will review police action.”

Authorities have said a plane was the target of the plot and an improvised explosive device was involved, without giving further details. Local media have alleged the men planned to use poisonous gas or a crude bomb disguised as a meat mincer.

Prime Minister Malcolm Turnbull described the plans as “advanced”.

The three other men remain in custody and have yet to be charged, after a magistrate late Sunday gave police an additional seven days to detain them.

Meanwhile, Etihad Airways announced on Tuesday that it was helping Australian police in their investigation into the plane plot.

“The Etihad Airways aviation security team is assisting the Australian Federal Police with its investigation and the matter is ongoing,” the Abu Dhabi-based carrier said in a statement.

“Etihad is complying fully with the enhanced security measures at airports in Australia and monitoring the situation closely. Safety is the airline’s number one priority.”

The airways made the statement amid a report from Sydney’s Daily Telegraph that one of the airline’s flights to Abu Dhabi was the target.

Australia’s national terror alert level was raised in September 2014 amid concerns over attacks by individuals inspired by organizations such as ISIS.

A total of 12 attacks, before the latest one, have been prevented in the past few years and 70 people have been charged.

Several terror attacks have taken place in Australia in recent years, including a Sydney cafe siege in 2014 which saw two hostages killed.