COLOMBO, Sri Lanka—The devastating Easter Sunday bombings in Sri Lanka were locally planned and executed, without direct guidance from the Islamic State militant group, investigators said.

Two Sri Lankan Muslim extremists learned how to build the explosive devices that killed more than 250 people in churches and hotels by studying Islamic State designs on the internet and conducting trial-and-error tests, including one that cost a bomb maker several fingers last year, people involved in the probe said.

One...