Scott Roeder, 51, of Kansas City, Kan., is charged with first-degree murder in the fatal shooting of Tiller May 31 at the church where Tiller served as an usher.
Roeder pleaded innocent and is being held in a Wichita jail on $20 million bail with trial scheduled for Sept. 21.
Federal agents, who declined to comment on an investigation, have spoken with many who have visited Roeder since the shooting, including two convicted abortion clinic bombers and several people who signed a declaration defending the killing of abortion providers, the Kansas City (Mo.) Star reported Monday.
"This guy has been in the hard-core anti-abortion circle for a long time, and there has been a pattern of communication and encouragement among these people," Eleanor Smeal, president of the Feminist Majority Foundation, said.
Supporters of Roeder deny a conspiracy, including Jennifer McCoy, a Wichita resident who visits Roeder, the Star reported. McCoy was convicted in the 1990s of trying to burn abortion clinics in Virginia.