A week ago, Sri Lankan tourist guide Ricky Costa was preparing for a typically easy Sunday ferrying backpackers between Colombo's tea shops and beach bars in his canary-yellow rickshaw. Then the blasts began.

The coordinated suicide bombings by Islamist militants at hotels and churches killed more than 250 people and sent shock waves through an Indian Ocean island state that had enjoyed relative peace since a civil war ended a decade ago.

How such a sophisticated operation could have been carried out in a country where violence by Islamist militants drawn from the Muslim minority was not high on the list of concerns has left Sri Lankans and foreign intelligence agencies stumped.