The letter, described as "burn after reading," was made public following a ruling by Florida Judge Danielle Brewer, raising significant questions about Roberta's knowledge and support for Brian's actions.
The doctor is charged with three felony counts of poisoning and one felony count of domestic battery with injury, the Orange County District Attorney's Office said in a statement.
The victims of the mass shooting have been identified as Evelyn Dieckhaus, 9, Hallie Scruggs, 9, William Kinney, 9, Cynthia Peak, 61, Katherine Koonce, 60, and Mike Hill, 61.