COLOMBO, Sri Lanka—Authorities here said an international terror group likely supported an obscure local Islamist group in carrying out a series of bombings that killed at least 321 people on Easter Sunday, weeks after the government had received warnings from the U.S. and India about a possible attack.

Government officials of this island nation said some of the suicide bombers and others arrested were linked to an obscure radical Islamist group known as National Thowheeth Jamath, a group the government singled out to police...