SYDNEY (Reuters) - Future “lone wolf” attacks in Australia will be difficult to stop despite heightened security in the aftermath of an Islamic State-inspired attack in Melbourne last week, a senior government minister said on Monday.

A Somali-born man set fire to a pick-up truck laden with gas cylinders in the center of Australia’s second-largest city on Friday before stabbing three people, killing a 74-year old man, in a rampage police called an act of terrorism.

Police said the man responsible for the attack, identified as Hassan Khalif Shire Ali, 30, was known to them because of his radical views but an earlier assessment concluded he posed no threat to national security.

Home Affairs Minister Peter Dutton, seeking to placate community frustration, said preventing similar attacks would be difficult despite heightened security.

“When you have a soft target, a place of mass gathering such as a shopping center or mall and you have a low level of sophistication like somebody grabbing a kitchen knife and picking up a couple of gas bottles - it is very hard unless you have direct evidence to stop,” Dutton told Australian Broadcasting Corporation radio.

Dutton said authorities were also hampered by the widespread use of encrypted messaging applications.

Australia proposed a law earlier this year that would require companies such as Facebook and Apple to provide access to private encrypted data linked to suspected illegal activities.

Prime Minister Scott Morrison said on Sunday the national terrorism advisory remained at “probable”, the midpoint of a five-tier system.

Authorities said Australia’s vigilance had helped foil at least a dozen plots, including a plan to attack Melbourne during Christmas in 2016 and a plan to blow up a flight from Sydney to Abu Dhabi using a bomb disguised as a meat mincer.

Two hostages were killed in 2014 during a 17-hour siege by a “lone wolf” gunman in a Sydney cafe.