The information came from “foreign intelligence,” the letter said. It did not specify which country had supplied the intelligence, but Indian security officials said they had given it to their Sri Lankan counterparts as early as April 4.

“We must look into why adequate precautions were not taken,” Mr. Wickremesinghe said on Sunday.

[Look at images from the devastation of the Easter Sunday attacks, and see how the country is mourning.]

An earlier arrest for one attacker

One of the suicide bombers was arrested just a few months ago, Sri Lankan officials disclosed on Monday, on suspicion of having vandalized a statue of Buddha. That is an inflammatory act in a Buddhist-majority nation where strident religiosity, on all sides, seems to be increasing.

The disclosure of the arrest came as Sri Lankan officials squared off over the attacks, and whether more could have been done to try to prevent them. In a government that is no stranger to crisis, the bitter recriminations suggested that a new one may be in the offing.

New details emerged about a confidential security memo on the group believed to be behind the attacks, which was issued 10 days before it struck. The memo appeared to lay it all out: names, addresses, phone numbers, even the times in the middle of the night that one suspect would visit his wife.