Because the nature of the crime is complex and they have not been able to get a solid lead yet.
The heist occurred on February 4 but the case was filed month later on March 15 with the Paltan police station but was immediately handed over to Criminal Investigation Department (CID).
Additional Deputy Inspector General Shaha Alam said that they are getting the necessary cooperation from the all the concerned foreign police forces.
As this is an international investigation, the process of gathering evidence is slower because several different police forces from several different countries are involved, CID sources added.
CID says that they are trying to apprehend both the mastermind and the hackers behind the heist.
Investigators are suspecting some 20 persons from four different countries to be involved in the heist and are unable to confirm the identities of those officials involved from Bangladesh Bank (BB) itself.
CID Special Superintendent Mirza Abdullahel Baqui said although they have information about some Bangladesh Bank officials being negligent, they do not have sufficient evidence connecting them to the heist.
Additional DIG told the Dhaka Tribune that their first priority is to arrest the criminals irrespective of their nationality along with recovering the stolen money.
Sources at CID said police from two countries where the cyber criminals were known to have stayed, has agreed to let them interview possible suspects.
However, the time and date of the interview is yet to be set.
The suspects from the two countries where the money was withdrawn are crucial to the investigation as an interview with them will reveal the mastermind behind the heist and confirm if any BB official is involved in the case.
Three teams from CID visited Sri Lanka and the Philippines but none of the possible suspects had been interviewed by the team during the visits.