Indian intelligence officers contacted their Sri Lankan counterparts two hours before the first attack.
Sri Lankan intelligence officials were tipped off by Indian intelligence officers about an imminent attack by terrorists hours before a series of suicide bombings killed more than 300 people on Easter Sunday, three sources with direct knowledge of the matter said.
Three churches and four hotels were hit by suicide bombers on Sunday morning, killing 321 people and wounding 500, sending shockwaves through an island state that has been relatively peaceful since a civil war ended a decade ago.
ISIS claimed responsibility for the attacks on Tuesday, without providing evidence of its involvement.
Indian intelligence officers contacted their Sri Lankan counterparts two hours before the first attack to warn of a specific threat on churches, one Sri Lankan defence source and an Indian government source said.
Another Sri Lankan defence source said a warning came "hours before" the first strike. One of the Sri Lankan sources said a warning was also sent by Indians on Saturday night.
The Indian government source said similar messages had been given to Sri Lankan intelligence agents on April 4 and April 20. Sri Lanka's presidency and the Indian foreign ministry both did not respond to requests for comment.