Investigators are trying to make sure that about the vital role played by thedetainees before making an official announcement.
According to sources Dey was not killed because of his work as a journalist, though the police remained tight-lipped about the whole issue.
"We have scrutinized Dey's articles on bookies, gambling, diesel mafia, environment and underworld, but none of the reports could have led to his death. He was shot at point-blank range. This indicates that someone desperately wanted Dey dead either due to personal enmity or vengeance,'' said an officer.
"We have made some headway, but have not come to any final conclusion," said Himanshu Roy, Joint commissioner of police (crime).
The police had earlier detained two suspects Mustafa and Sikander from Surat, Gujarat on Jun 23.
Dey who was an investigative journalist in the MiD DAY was killed by four unidentified assailants on Saturday, Jun 11.
Police earlier had arrested four people suspecting their connection with Dey's murder, though they were released later.
According to a report in the Times of India, Mumbai police succeeded to access Dey's emails, SMSs and notes and received some clues with possibility of giving them vital clues in the whole case.
The Bombay High Court had given an ultimatum to the police to present its report by Jul 6 to ascertain the progress made in the investigations. After repeated requests, the case has not yet been handed over to the CBI.