It's still early in the investigation of Nisman's death on Sunday. The indications available in the public record at this point strongly indicate suicide. As of the last report I saw, there has been no suicide note or message found.
This is a guide to recent events in the Nisman case, Five key days: from accusations to tragedy Buenos Aires Herald 01/20/2015
Horacio Verbitsky discusses various aspects of the case here, La dudosa muerte del fiscal Página/12 20.01.2015.
He also appeared on TV Pública argentina to discuss Nisman's death, Visión 7-Verbitsky: "Hay una relación promiscua entre jueces y servicios de inteligencia" 19.01.2015:
And a second piece with Verbitsky, Visión 7 - Horacio Verbitsky sobre la muerte de Nisman 20.01.2015
Verbitsky cautions against jumping to quick conclusions about whether this was a suicide or not.
The Centro de Información Judicial (CIJ) has made Nisman's report with his accusations against President Cristina Fernández, her Foreign Minister Hector Timerman and three other politicians associated with Cristina's Peronist party available for download: Denuncia completa (75812.02 Kb.).