One of them smiled as they travelled up to the buffet restaurant in a lift.
As guests tucked into breakfast he enters wearing a backpack, before detonating the device.
At least 321 people were killed and 500 wounded in the series of suicide bombings that ripped through churches and luxury hotels on Easter Sunday.
The vast majority of the victims were Sri Lankan but the dead but others hailed from the US, Denmark, China, Japan, Portugal and Australia.
Police have arrested 60 people in connection with the bombings.
Isis has claimed the attack which it said in a statement was carried out by “fighters of the Islamic State”.
The group released images of eight men in black carrying knives, their faces covered, posing in front of an Isis flag, as well as a video in which they pledged allegiance to the Isis leader Abu Bakr al-Baghdadi.
Sri Lankan authorities are investigating whether the cell which carried out the attack, previously identified as a local outfit named National Thowheed Jamath, was aided by returning fighters from Iraq and Syria.