The deceased hailing from Bihar are Papu Kumar Singh (25), Shamsher Mantri (20) and brothers Ahot Bihari (20) and Raj Bihari (19), police sources said.
The injured, also from Bihar admitted to Bongaingaon Civil Hospital, are Amol Gupta, Ramanand Ojah and Babulal Karmakar.
The militants stopped the bus coming from Siliguri in West Bengal to Shillong in Meghalaya at about 10 PM on NH-31 in Ramphalbeel area under Serfanguri police station.
The ultras asked the eight passengers to come out and then lined them up before opening fire on them from close range killing five on the spot, they said.
District Additional Superintendent of Police S Panswar has said operations were intensified to nab the culprits and security measures have been stepped up.
Stating the cause of the attack was not yet unknown, Paneswar said there were no prior inputs about such an attack.