CANBERRA, Australia — The Islamic State plot to carry out random beheadings in Sydney alleged by police is a simple and barbaric scheme that has shaken Australians. But terrorism experts on Friday questioned whether the ruthless movement had the capacity or inclination to sustain a terror campaign so far from the Middle East.

Police said they thwarted such a plot by detaining suspects and raiding more than a dozen properties across Sydney on Thursday.

The Islamic State militant group has beheaded three Westerners in the Middle East in recent weeks and recorded the brutal slayings to make propaganda videos widely condemned.

Two of the 15 people whom police had detained Thursday have been charged. Nine were freed before the day was over, and the rest released on Friday.

Prime Minister Tony Abbott conceded it was difficult to safeguard the Australian population against such attacks.

“The regrettable reality is that to mount the kind of attacks which ISIL in Syria and in Iraq has in mind for Australia, all you need is a determined individual who will kill without compunction, a knife, an iPhone and a victim,” Abbott told Seven Network television on Friday, using one of Islamic State’s former names, Islamic State of Iraq and the Levant.

Some terrorism experts saw the plot as a potential shift in Islamic State’s focus from creating an Islamic caliphate in the Middle East. Others said it is more likely a symptom of policy confusion within a disparate group.