The carnage in Sri Lanka, one of the worst terrorist atrocities this century, could signal the start of a new campaign of "spectacular" attacks around the world.
There have long been warnings that Islamic State militants, many trained in explosives and guns after time spent fighting in Syria and Iraq, could attempt such a move following the collapse of their so-called caliphate last month .
Al-Qaeda and its affiliates similarly have the know-how to direct a coordinated barrage of suicide bombings in multiple locations, as happened in Sri Lanka on Easter Sunday .
But no international terrorist group has yet claimed any involvement, which is unusual.
The 9/11 attack on the US by al-Qaeda was the most notorious "spectacular" act of Islamist terrorism in living memory.
Other high-profile incidents include the Mumbai attacks in 2008, carried out by a militant Islamist group called Lashkar e-Taiba, in which more than 170 people died, and the Bali bombings of 2002, again by Islamist extremists, which killed more than 200 people.
Islamic State was linked to the bombing of Coptic Christian churches in Egypt on Palm Sunday two years ago , in which dozens of worshippers perished.
The Sri Lankan authorities have named a little-known local Islamist group as being behind the bloodshed in churches and at hotels at this popular holiday destination, caused by at least seven suicide bombers.
But there is no doubt that international terrorist expertise played a part in helping National Thawheed Jama'ut, also known as NTJ, if they are proven to have been the culprits.
The local group previously had only been linked to vandalising Buddhist statues - a long way away from killing and maiming hundreds in a coordinated attack involving multiple targets at the same time.
Sri Lanka is struggling to understand how this could have happened on its soil.
There are signs that warnings may not have been sufficiently acted upon.
The country's police chief is said to have issued a warning on 11 April - nine days before the bombers struck - saying a "foreign intelligence agency" had reported a little-known local Islamist group was planning attacks on churches and the Indian high commission.
It is not known what action this tip-off generated within the police.
However, Prime Minister Ranil Wickremesinghe had not been made aware of the report, according to a fellow minister.
There is a rift between him and the president, which sparked a political crisis last year. The prime minister has since been kept out of intelligence briefings, which could be why he was unaware of the warning.
The police say they plan to investigate whether any intelligence reports of the possibility of an attack were missed as part of their wider investigation into what happened.
British, US and other intelligence agencies will be working hard to identify possible links between the Sri Lanka bombers and any known foreign individuals or groups.
They will be aware that other similar attacks could be just a matter of time.