The Improvised Explosive Device is understood to have been planted days before today's incident and was triggered by the naxals armed with real-time intelligence that Special Police Officers (SPOs) were travelling in the civilian bus.
The Left Wing Extremists used the technique to blow-up a civilian bus today in which at least 40 people were feared killed, officials said.
They attacked by digging a tunnel on either side of the road to reach the concrete top from below to plant the IED, they said.
"In this way, they do not disturb the crust so to avoid any suspicion. The signs of dredging on sides of road can be easily wiped off. Also, in this way they can quickly fit an IED and carry out the blast in the most effective way," they said.
In a similar use of IEDs last week, the Maoists had blown up an armoured vehicle in which CRPF personnel were travelling killing eight.
Top police officials said the same technique was used by the LTTE against the Lankan forces.
