COLUMBIA, S.C. (AP) – Authorities say a white man charged with the shooting deaths of nine black churchgoers in Charleston “self-radicalized” in the months before the shootings and grew more entrenched in his beliefs in white supremacy.

The information was filed Monday as part of a listing of more than a dozen expert witnesses that prosecutors intend to call in Dylann Roof’s federal death penalty trial later this year.

The 22-year-old Roof is charged in the June 2015 deaths of nine black parishioners at Emanuel AME Church.

Authorities have long said they believed Roof was the author of an online manifesto in which he embraced Confederate symbols and talked about white supremacy. Handwriting analysis experts are expected to testify that he wrote another similar document in jail.