Mr Dey was killed, allegedly upon Rajan's instructions, in suburban Powai on June 11, 2011.
Gangster Chhota Rajan, prime accused in the journalist J Dey murder case, thought that the veteran crime reporter was working for his rival Dawood Ibrahim and ISI, according to the charge sheet filed by CBI.
Mr Dey was killed, allegedly upon Rajan's instructions, in suburban Powai on June 11, 2011.
According to CBI, Rajan told one Manoj on phone that journalist Jigna Vora (a co-accused) used to tell him "repeatedly" that Dey "is writing wrong things and is in touch with the other gang", and was a traitor.
Transcripts of Rajan's telephonic conversations are part of the charge sheet filed by CBI in a local court last week.
Rajan further said that Mr Dey was not a good person, and was working for Dawood and ISI. Jigna used to tell him that Mr Dey was a "traitor", he said.
Mr Dey's wife told CBI that for a month before his murder, he was very upset. "Once he told me that he was going to die but not due to illness," her statement reads.
According to Mr Dey's mother, around 15 days before his death he had told her that he would be visiting the Philippines. A few days before his death she also received a call from the accused Vinod Chembur (who later died) enquiring about Mr Dey.
Fifteen days prior to the murder Mr Dey had signed agreement with actor Ajay Devgn for writing a film script, she said.
The charge sheet also contains statements of journalists with whom Rajan spoke on phone after Mr Dey's murder. Rajan told one of them that he regretted killing Mr Dey.
"A news reporter has to remain in his limits and work ...if you are writing lies..you are inviting serious trouble," Rajan said.