COLOMBO, Sri Lanka — Nine suicide bombers from mostly educated, middle-class backgrounds carried out the attacks in Sri Lanka that killed more than 350 people on Easter Sunday, the authorities said on Wednesday as they warned of an ongoing terrorist threat and continued making arrests.

The bombers, one of whom was a woman, were all Sri Lankan, officials said. But the authorities were continuing to investigate whether the Islamic State, which on Tuesday claimed responsibility for the coordinated blasts, had provided more than symbolic support, such as by training the attackers or building the bombs.

Eight bombers struck at hotels and churches across the island on Easter Sunday. During a subsequent police raid near Colombo, Sri Lanka’s capital, a female suspect also blew herself up in front of two of her children, killing them all, along with several police officers, investigators said.