Australian Prime Minister Tony Abbott said Thursday that a siege at a Sydney café earlier this week that claimed the lives of two hostages and a gunman, could have been prevented. The government was widely criticized for failing to monitor the gunman who held 17 people hostage for nearly 16 hours.

Man Haron Monis, the 50-year-old self-described Muslim cleric, had been charged with being an accessory to his former wife’s murder, sexual offenses and other crimes, but was freed on bail. He was also on the national security agency’s watch list, but later dropped off of it for unknown reasons.

"This has been a horrific wake-up call," Abbott told Macquarie Radio, according to The Associated Press. "This was an atrocity -- it may well have been a preventable atrocity, and that's why this swift and thorough review is so important."

Abbott has reportedly ordered a transparent government review into the events that led to the siege, including why Monis was out on bail despite having a long criminal history, and how he obtained a shotgun.

In 2011, Monis’ former wife Noleen Hayson Pal told police that he threatened to kill her. He was subsequently charged with stalking and for activities that caused fear of physical or mental harm. He was later found guilty of the charge. Pal reportedly testified in January 2012 that Monis told her: "If I don't get to see the boys more than I am seeing them now, I'll make sure you pay for it -- even if it means I have to shoot you."

Pal was reportedly stabbed to death and set on fire after Monis was charged. Police charged Monis and his then-partner, Amirah Droudis, with Pal's killing.

Monis was also reportedly convicted and sentenced to 300 hours of community service in 2013 for sending "grossly offensive" letters to the families of soldiers who were killed in Afghanistan between 2007 and 2009.

According to AP, three days before the siege, Australia's high court refused to hear Monis’ appeal of the convictions for sending the letters.

"Just like about everyone else from the premier down, I was incredulous and exasperated at this," Abbott reportedly said. "This guy has a long history of violence, a long history of mental instability, he has a long criminal record and obvious infatuation with extremism. It was extraordinary he was on our streets."