Some members of the Sri Lankan government claim the attack that killed 321 people in their country (and injured hundreds more) is a retaliation for the attack against the Muslim targets in Christchurch where 50 people were murdered (and dozens more injured).
In addition, Islamic State has also now claimed responsibility for the Sri Lanka attack and released pictures of the alleged ring-leader, as well as what it has claimed was the nom de guerre of each attacker.
The group said the bombers targeted citizens of the US-led coalition fighting ISIS and referred to Easter as an "infidel holiday".
The theory that ISIS was involved, and this is somehow a payback for what happened to the Muslim community is supported by three ideas.
First, reprisals in situations where there is war without rules are common. This means, one illegal act follows another, in a way to retaliate, or warn off, the opposition. Working on that idea, an attack on sanctuaries of one religious group is paid back with an attack on the sanctuaries of the other. The date of Easter adds to the symbolism of this attack, and also fits this scenario.
The second reason why this retaliation theory works is that calls for retribution following Christchurch, including from ISIS, were widely reported. This international linkage is possible, as aside the calls for retaliation, radical jihadi groups, such as ISIS or al Qaeda, have form in terms of attacking places of religious sanctuary in other countries.
The third, and most convincing, reason that external groups like ISIS or al Qaeda could be involved is because what was happening in Sri Lanka prior to the Easter terror attacks does not fit the profile of what could be expected. That is, the local jihadi groups involved in trying to cause strife previously spent much of their efforts attacking effigies of the ruling (Buddhist) religion.
However, something clearly changed in terms of targets and capability. Within a remarkably short space of time their target swapped from Buddhists to Christians (and foreigners) and in addition, they seem to have achieved a quantum leap from acts of vandalism to sophisticated terror operations involving eight synchronised bombings with military grade explosives.
Despite all of these reasons suggesting that there may be linkages between Christchurch, ISIS and Sri Lanka, there are four considerations which invite scepticism.
First, although ISIS has claimed responsibility, the fact that a terror group claims responsibility is not alone enough for attribution, as there is often a surplus of claimants as many such groups try to claim the work of others.
Accordingly, evidence is required.
What does not add up here is that if the attack in Sri Lanka really was a reprisal for Christchurch, then they probably would have mirrored the use of social media to a much greater degree, to which organisations like ISIS were renown. Even if the Sri Lankan authorities were quick at closing down many social media platforms, live streams like Christchurch would have gotten out before the switch was turned off.
The second reason to be sceptical of the ISIS-Christchurch linkage is the short space of time between the attacks in Christchurch and then Sri Lanka. Everything from collecting explosives, surveillance and target selection, through to recruitment, training and handling of suicide bombers, takes time.
Even the number of nationals from Sri Lanka who may have fought for ISIS in the Middle East (perhaps 40) would not create a clear pipeline of volunteers. Most (but not all) who return home lose their desire to keep fighting. Only when the national origin of the suicide attackers becomes clearer, will be it possible to work out how much of this was driven from offshore, or the onshore risk was considerably under-estimated.
The third set of considerations suggesting a disconnect in the ISIS-Christchurch idea is, according to our Prime Minister, our intelligence agencies have no proof of such assertions that the attack in Sri Lanka was related to Christchurch.
This statement needs to be taken very seriously, as our agencies are linked to the Five Eyes, which are then interlinked with other, friendly, intelligence agencies, of which th relationship between Indian and American security intelligence is notable.
Finally, it should be noted the Sri Lankan authorities have reason to blame Christchurch. This distances culpability, in as much as the perpetuator of what happened in ours, was Australian.
In many ways, the answer to the above equations do not matter. What matters is innocent people were murdered in places of sanctuary. The purpose of such attacks, irrespective of the culprits, is to provoke similar acts of aggression and reprisal. They want more hatred, anger and violence. This is exactly what the mass-murderer in Christchurch desires.
That means security everywhere will need to be stepped up to prevent all flavours of extremists from trying to continue such lunacy in the future.
• Alexander Gillespie is a professor of law at Waikato University.