The first grenade exploded on January 15 at the army office adjacent to the Raj Bhavan. The second blast followed within minutes, a few meters away, injuring a woman.
Biren said that apart from being a high security area there are sentry turrets in the army office.
Two more grenades exploded on Monday night at the Polo Ground in the heart of Imphal city, injuring two troopers.
The Chief Minister said that the safety pins of the grenades were found within the boundary wall.
Official speculation is that these grenades were not thrown from outside the wall, he said.
"From the CCTV footage, it was established that there was no movement of people near the Polo Ground at the time of explosions," Biren added.
On Tuesday, nine troopers were injured in a twin blast.
Unidentified persons lobbed two hand grenades over the wall which "exploded about inside the transit camp" of Assam Rifles.
This is something "incomprehensible" to the government, he said, adding "under these circumstances we approached the higher authority for an NIA probe and it was conceded to."
Sources told IANS that Biren had urged Home Minister Rajnath Singh for a NIA probe into the January 15 blasts at the army camp.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
