A case was filed on Sunday against Gorkha Janmukti Morcha (GJM) chief Bimal Gurung and four others for their alleged involvement in grenade attack which killed a civic volunteer on Friday night.
A hand grenade was hurled at the Kalimpong Police Station at around 11 pm on Friday, the police said.
A high intensity blast had also taken place in Darjeeling town in the early hours of Saturday.
A civic police volunteer was killed on the spot while the two other police personnel were admitted to a nearby hospital with severe injuries.
The situation in Darjeeling remained tensed as the indefinite shutdown in the hills entered its 67th day on Sunday.
Barring medicine shops, all other commercial establishments, schools and colleges remained shut in the Darjeeling hills.