The terrorist attacks, most of which seem to have been carried out by suicide bombers, were clearly well coordinated.

There were six explosions in the initial attacks, followed by another two afterwards during the police search for suspects.

One of the explosions was at a housing complex in the Dematagoda suburb of Colombo, where police engaged in a firefight with suspected terrorists.

Police reportedly found explosives in at least one apartment, which may have been where the attacks were planned.

Three police officers were killed when an improvised explosive device was detonated by one of the suspects.

There does not seem to have been much, if any, protective security at the churches or hotels targeted.

One of the targets, the Shangri-La hotel restaurant, was on the third floor of the hotel, which means the suicide bomber was able to come off the street, access the lobby, and make his way to the third floor without being security screened.

Since the defeat of the Tamil Tigers in May 2009, Sri Lanka seems to have become complacent about any terrorism threat.

There were nine terrorism-related deaths in 2010, one in 2011 and four in 2014. Since then there had been no deaths from terrorism.

Sri Lanka has however seen some sporadic religious violence.

In March 2018 a state of emergency was declared after members of the majority Buddhist Sinhalese community attacked mosques and Muslim-owned properties.

This followed claims on Facebook that Muslims were responsible for defacing Buddhist statues. Buddhist hardliners have also been harassing Christians.

So far, no one has claimed responsibility for the East Sunday attacks, but 13 people have been arrested according to Sri Lankan authorities, and the government says it has identified the attackers - who are being described as "religious extremists".

The country's Defence Minister, Maithripala Sirisena, has said that they "believe these were coordinated attacks, and one group was behind them".

Sri Lanka's senior police officer, Inspector General of Police Pujuth Jayasundara, put out a police alert 10 days before the terrorist attacks.

The alert stated: "A foreign intelligence agency has reported that the NTJ (National Thowheeth Jama'ath) is planning to carry out suicide attacks targeting prominent churches as well as the Indian high commission in Colombo".

It is not clear if any precautionary measures were taken or if the group played any part in Sunday's violence. (The NTJ is a local radical Muslim group that was linked to the Buddhist statue defacement.)

On Sunday, Prime Minister Ranil Wickremesinghe pointedly said he had not been informed of the alert.

Sri Lanka is predominantly a Buddhist country. Theravada Buddhism is Sri Lanka's largest religious group, involving 70.2 per cent of the population, according to the latest census, and is the religion of Sri Lanka's Sinhalese majority. It is given primary place in the country's laws and is mentioned in the constitution.

Most of the Tamil population are Hindus. Hindus make up 12.6 per cent of the population and Muslims 9.7 per cent. Sri Lanka also has about 1.5 million Christians, the vast majority of whom are Roman Catholic.

Islamist extremism has not been a security problem in the past in Sri Lanka, unlike the situation in neighbouring India.

The official list of 37 extremist organisations active in Sri Lanka does not contain any Buddhist or Islamist groups. Nearly all of the groups listed are Hindu Tamil, with a small number of extreme left-wing groups.

The scale of this attack seems well beyond anything Buddhist extremists could contemplate.

While the Hindu Tamil Tigers often used suicide bombers, the targets in this attack - hotels, churches and foreigners - are more consistent with the kind of targets chosen by Islamist al-Qaeda, Islamic State, and their affiliates.

Tamil Tiger attacks were invariably directed against Sri Lanka government targets and individual politicians. Because of the Tigers' reliance on their overseas support network, they never targeted foreigners.

Sri Lanka's inexperience with Islamist extremism could have made Sri Lanka an ideal target for a local Islamist group seeking to cause maximum impact.

These attacks were meant to create a high death toll and draw international attention.

"So far the names that have come up are local," the Sri Lankan Prime Minister commented at a media conference, noting that they were still looking to see if any foreign elements were involved.

A particular security concern would be any involvement of some of the "scores" of Islamic State fighters returned from Syria and Iraq since 2017.

In January 2019, police raided an Islamic State chapter's training camp at remote Wanathawilluwa, and recovered substantial quantities of explosives, said to be intended for blowing up Buddhist monuments in the ancient city of Anuradhapura.

Clive Williams is a visiting professor at the ANU's Centre for Military and Security Law. He has previously worked with security authorities in Sri Lanka.