"Some of them have been identified and attempts are on to identify the rest," he told journalists here.
Four secular bloggers and a publisher have been hacked to death this year by suspected Islamists in a similar manner but police investigations have not made much progress except for coming up with "some names".
The minister said trials would be held for all the killings.
"Chargesheets have been submitted in two cases already and are under process in two more," he was quoted as saying by BD News.
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The recent killings of freethinkers are part of a conspiracy to destabilise the country, he said.
"It is part of local and foreign conspiracies. War crimes trials are on in the country. These killings are being carried out to foil the trials," Kamal said without elaborating.
The law enforcement agencies had been kept on the highest alert, he said.
Faisal Arefin Dipan, 43, a Bangladeshi publisher who worked with slain atheist writer and blogger Avijit Roy, was killed last week in his third-floor office here.
Before Dipan's murder, four secularist bloggers were killed by Islamist militants in Bangladesh this year.
Bangladesh-born US writer Roy was hacked to death by unidentified assailants on February 26. Blogger Washiqur Rahman was murdered in central Dhaka on March 30. Writer and blogger Ananta Bijoy Das was killed in a similar attack in northeastern city of Sylhet on May 12.
Ahmedur Rashid Tutul, a publisher who worked with Roy and bloggers -- Ranadipam Basu, 50, and Tareque Rahim, 30, -- were hacked by unidentified assailants in a separate incident last week.
A group identifying itself as Ansar al-Islam -- Bangladesh chapter of al-Qaeda in Indian Subcontinent (AQIS) -- claimed responsibility for the attacks.
Two foreigners -- Cesare Tavella and a 66-year-old Japanese farmer have also been killed in the last two months. There murders were claimed by IS.