Vahedkhon Quassidinov, a key figure in the IRP, told RFE/RL's Tajik Service that he believed Marufov was pushed to his death.
Deputy Interior Minister Abdulrahim Qahhorov told RFE/RL that the circumstances of Marufov's death remained unclear.
The IRP's Quassidinov says Marufov's religious beliefs would have prevented him from committing suicide.
Marufov died on May 4, one day after being arrested in the northern town of Isfara.
He also said that it remains unclear why Marufov had been detained on May 3.
An investigation in still under way.