Original sentence: Tappe states that individuals responsible for school shootings were indigo children.The promotion of the concept by Tober and Carroll brought greater attention and exposure to the topic, and their book became the authoritative source on indigo children
Edited sentence: Nancy Anne Tappe states that individuals responsible for school shootings were indigo children.Since then they co-authored two other books on indigo children.The development of that theory by Jan Tober and Lee Carroll brought greater attention and exposure to the topic, and their book became the authoritative source on indigo children