"We do not think these attacks were carried out by a group of people in the country," said Sri Lanka government spokesman Rajitha Sinaratni, "an international network that could not have been helped without such assistance."
A statement by the Sri Lankan government accused a "local (Islamic) group of being behind the" Easter attacks ".
On Sunday, it targeted a series of attacks by two churches, four hotels in and around Colombo, and a third church on the northeast coast of the country.
The bombings killed 290 people and injured 500 others, and authorities arrested 13 people for their involvement in the operations.