SYDNEY - Australian Prime Minister Tony Abbott said on Tuesday the gunman behind the Sydney cafe siege, in which two people were killed, was well known to authorities and had a history of mental instability.A police source named the gunman as Man Haron Monis, an Iranian refugee and self-styled sheikh facing multiple charges of sexual assault as well as being an accessory to murder."The perpetrator was well known to both state and commonwealth authorities," Abbott told reporters in a brief press conference in Canberra. "He had a long history of violent crime, infatuation with extremism and mental instability.""These events do demonstrate that even a country as free, as open, as generous and as safe as ours is vulnerable to acts of politically motivated violence," Abbott said.Speaking in Sydney later in the day alongside New South Wales Premier Mike Baird and Australian Federal Police Commissioner Andrew Colvin, the prime minister criticized the situation but also commended both the police response as well as that of the public.He described Monis as a "deeply disturbed individual" who was "consistently weird.""How can someone who has had such a long and checkered history, not be on the appropriate watchlists?" Abbott said. "And how can someone like that be entirely at large in the community?"These are questions that we need to look at carefully and calmly and methodically, to learn the right lessons, and to act upon them," he said.Abbott spoke about the difficulty of preventing attacks of this sort."Even if this individual, this sick and disturbed individual, had been front and center on our watchlists, even if this individual had been monitored 24 hours a day, it's quite likely, certainly possible, that this incident could have taken place, because the level of control that would be necessary to prevent people from going about their daily life, would be very, very high indeed."The prime minister next turned his focus to the worldwide response to the siege. "Tens if not hundreds of millions of people right around the world have been focused on the city of Sydney which has been touched by terrorism for the first time in more than 35 years."Abbott commended the New South Wales police that dealt with the situation, while also praising the public's resilience immediately after."I think every Sydneysider can feel quietly proud of the way this city has handled one of the most difficult 36 hours in our history," he said."People have responded with typical Australian decency and generosity and the spontaneous shrine which has developed now in Martin Place is so much an expression of the innate goodness and decency which is a mark of the Australian character."