Bombing in Sri-Lanka's churches and tourist hotels took lives of 310 people and left 500 people injured. The tragedic event resulted the declaration of "Mourning Day". The Sri Lankan leader said that the country is mourning as one, days after bombs ripped through eight locations in three cities in the Indian Ocean island nation, CNN reported.

"Today as a nation we mourn the senseless loss of innocent lives this past Easter Sunday," he tweeted.

"I would like to thank the military and police forces, the medical personnel and all those who have worked bravely and tirelessly without concern for their own safety, to ensure the safety and security of our citizens. It is imperative that we remain unified as Sri Lankans in the face of this unspeakable tragedy."

Warning of potential threat

The potential threat was ignored by governmental circles.

When suicide bombers walked into three churches around Sri Lanka, and three upscale hotels in Colombo, they faced no enhanced security. As worshipers closed their eyes in prayer, as hotel guests lined up for breakfast, the attackers detonated their devices -- and the effects were devastating.

Five days later, on April 9, the country's Defense Ministry informed the Inspector General of Police of this alleged plot, and named a group believed to be behind the plan, the Nations Thawahid Jaman (NTJ). Unusually, the memo also included a list of suspects.

Demanding serious action as to why the warning was ignored, Sri Lankan Minister of Telecommunication, Harin Fernando, said, "Some intelligence officers were aware of this incident. There was a delay in action. What my father heard was also from an intelligence officer. Serious action needs to be taken as to why this warning was ignored."

According to CNN, US and India also warned Sri-Lanka about the potential attacks on churches and tourist destinations.

Police Media Spokesperson SP Ruwan Gunasekara told CNN that “around 40 suspects” have been arrested in connection with the Easter Sunday bombings.

All of those in custody are Sri Lankan, he added.

Nations Thawahid Jaman

Islamist grop called The Nations Thawahid Jaman is the main suspected group over the terror attack.

What is puzzling about the attacks is that while Sri Lanka has seen plenty of terrorism in the past -- a result of the civil war between the separatist Tamil Tigers and the Sri Lankan government -- that war ended a decade ago. And the Tamil Tigers had notably focused their attacks on "transit hubs, Buddhist shrines, and office buildings ... and Sri Lankan officials," rather than churches, according to a Council on Foreign Relations report.

There were doubts that the NTJ would have had the capacity to carry out such a sophisticated and coordinated attack alone. Transnational Islamists are known to operate in places like Pakistan, Malaysia and the Philippines. It is unclear whether the details contained in the warning matched the atrocity that eventually took place on Sunday.

While no group has claimed responsibility, Sri Lankan Defense Minister Ruwan Wijewardene told reporters the "terrorist incident" was carried out by those following "religious extremism."

"We have not had any separatist movements in the past 10 years and this came as a shock to all of us," he said.