His brother-in-law Ali Haider, arrested on Friday, is an accused in the Rajan murder case.
Jalalabad Police station OC Akhter Hossain said Amin allegedly threatened Rajan’s father for filing the case.
He also helped in carrying away the body, the OC said.
Rajan was tortured to death at Kumargaon on suspicion of theft on July 8. The torturers filmed the incident and posted it on a social networking site.