NEW DELHI—An alleged follower of Zahran Hashim—the suspected organizer of the deadly Easter bombings in Sri Lanka—was in custody in India after investigators said he conspired to launch a suicide attack in the southern state of Kerala.

The Indian national, identified as 29-year-old Riyas A, also known as Riyas Aboobacker, was arrested by the National Investigation Agency late Monday and was sent to judicial custody, or jail, Tuesday after appearing in a local court.

Mr....