More than a week after the Easter Sunday massacre in Sri Lanka, a devastatingly well-coordinated assault that targeted churches and hotels around the country, the shock still lingers.

It was by far the deadliest attack to strike Sri Lanka since the dark days of its brutal 26-year civil war, which ended in 2009. And it shattered the relative stability that had prevailed in the country in the subsequent decade.

Information provided by Sri Lankan officials has linked a small and little-known extremist outfit named National Tawheed Jamath (NTJ) to the attack. Indeed, NTJ is no jihadist juggernaut; it's an entity mostly known for defacing Buddhist statues.

It's downright terrifying that such a modest and under-the-radar organization previously known for little more than vandalism could pull off such a sophisticated and catastrophic attack.

In recent years, similarly complex and well-choreographed mass-casualty assaults across the globe have been carried out by the household names of the terrorism world -- al-Qaeda, ISIS, Lashkar-e-Taiba, al-Shabab, Boko Haram, the Taliban, and so on.

This is why there’s good reason to believe that ISIS, which took credit for the attacks two days after they happened, played a major role in the tragedy—even though there is no evidence (aside from ISIS boasts of responsibility) of ISIS complicity.

To be sure, at this moment in time, one should always take with some healthy grains of salt any ISIS claim of responsibility for an attack.

ISIS is down on its luck these days, having lost its territory in Syria and Iraq and trying to regain its footing. With ISIS on the defensive, it has a strong incentive to take credit for an attack that it didn’t actually carry out—simply in order to demonstrate that it’s still a potent force to be reckoned with.

Typically, ISIS claims credit within hours if not minutes of the attack.

Another reason to be skeptical of ISIS’s claim in this particular case is that the group took so long to take responsibility. Typically, ISIS claims credit within hours if not minutes of the attack. The two-day lapse in this case is perplexing. One possible explanation for the delay is that ISIS was expecting other attacks to take place—and Sri Lankan police did indeed uncover additional bombs and other weaponry in the days after the tragedy—and didn’t want to speak up until the whole catastrophic operation had been completed.

All this said, it beggars belief that such a small Islamist militant group could carry out such a sophisticated strike. And there’s no candidate other than ISIS that we could reasonably expect to have swooped in to help. Al-Qaeda has been badly degraded, and its South Asia affiliate has little reach beyond Afghanistan, Pakistan, India, and Bangladesh. Meanwhile, Pakistan's Lashkar-e-Taiba and Bangladesh's Jamaat-ul-Mujahideen Bangladesh have enjoyed cross-border reach. But their capacity and desire to assist with such a large-scale attack is questionable.

The other reason to believe ISIS was involved was the targeting. Christians have rarely come under attack in Sri Lanka, and the small number of Islamist extremists that exist there have not had problems with them. If local Islamists militants in Sri Lanka were to have problems with any community, it would be Buddhists.

Indeed, in recent years, Buddhist extremists have staged episodic attacks on Muslim targets. So if an Islamist radical group wants to stage an attack in Sri Lanka, one would expect it to hit out at the Buddhists with whom Muslims have clashed—and not the Christians targeted in the strikes.

Targeting Christians—and foreigners, another group that rarely comes under attack in Sri Lanka—is a hallmark tactic of international jihadist terrorists like ISIS.

India has claimed that it learned of the Sri Lanka plot while interrogating several ISIS sympathizers on its soil several months ago.

From the facts that have emerged to this point, the most likely explanation is simple: NJT, or, more likely a more radical fringe element of NJT, was quietly able to tap into small pockets of radicalized constituencies in Sri Lanka, and slowly develop the capacity and resources to strike with some major assists from ISIS. Links could have been made from the 32 Sri Lankans estimated to have fought for ISIS in Syria and Iraq, or from connections to ISIS or al-Qaeda figures in South Asia. India has claimed that it learned of the Sri Lanka plot while interrogating several ISIS sympathizers on its soil several months ago.

Unfortunately, thanks in no small part to Sri Lankan government dysfunction, this plan was executed with little resistance—and quite successfully. Media accounts have painted an alarming picture of Sri Lankan authorities failing to act on multiple advance intelligence reports that underscored the threat of NJT-patented bombings. This included Sri Lankan officials being presented with information that listed the names of potential suspects—some of whom actually staged the attacks—and yet nothing was done.

An ongoing political feud between Sri Lankan President Maithripala Sirisena and Prime Minister Ranil Wickremesinghe may have contributed to people on high levels not knowing about this intelligence. So there is the very real risk that Sri Lanka’s government was so distracted by its own dysfunction that it proved unable to anticipate and defuse a catastrophic threat, despite ample and highly specific warnings.

Sri Lanka's government now faces a major reckoning. It must confront a dramatic new terrorist threat, address a fresh political crisis brought on by its multiple intelligence failures, and bring a semblance of unity and calm to a grieving nation at risk for reprisal attacks and other societal unrest. Complicating all of this is the fact that Sri Lanka has an election later this year.

These three objectives constitute a tall order for any government, but especially for one riven with high levels of division and dysfunction--the very qualities that likely helped a small terror group seemingly come out of nowhere to pull off an unthinkable act with the assistance of an international militant group that may be down but is most certainly not out.

This post is an updated and expanded version of a short essay published by CNN.com on April 23.  Image: AP.

Follow Michael Kugelman, deputy director of the Asia Program and senior associate for South Asia, on Twitter @MichaelKugelman.

The views expressed are the author's alone, and do not represent the views of the U.S. Government or the Wilson Center. Copyright 2019, Asia Program. All rights reserved.