Grohl also approached death metal legend Chuck Schuldiner of Death to contribute to Probot. However, Schuldiner was struggling with brain cancer and was unable to be involved despite the efforts of Grohl and others to raise funds for his medical treatment. Grohl also attempted to get Slayer's lead singer Tom Araya on the album, but he was unable due to scheduling conflicts. To replace him, Grohl invited Kurt Brecht from D.R.I. Grohl mentioned he and Sweeney had discussed and considered a lot of different singers, including the retired Jeff Becerra of Possessed, Chuck Billy from Testament, Pantera's Phil Anselmo, and the vocalists from Kreator, Destruction, Hirax and Candlemass. Sweeney vetoed Unleashed's Johnny Hedlund, who at the time was rumored to be a Nazi sympathizer.