To learn more about Mary Surratt read: The assassin's accomplice : Mary Surratt and the plot to kill Abraham Lincoln / Kate Clifford Larson.
The author started her research in an attempt to prove the innocence of Madame Surratt; her research revealed to her the truth: that Mary Surratt was guilty as hell.
The plot to kill Abraham Lincoln was probably launced in return for this.
http://en.wikipedia.org/wiki/Dahlgren_Raid
Mary Surrat was most certainly guilty and complicit.