Banka Superintendent of Police Satya Prakash said nine labourers were injured in the incident, of which three have bullet injuries.
The torched vehicles include two SUVs, two tractors, an excavator, an earthmover and a motorcycle.
"The incident happened near Dudhiyatadi village along Odni river where a bridge is being constructed by Chaubatiya Constructions. It seems to be a fall out of levy (extortion) demand by Maoists," Prakash said.
The SP said around 20 Maoists arrived at the site searching for a particular official of the company. When they did not find him, they assaulted those working there leading to injuries to six.
Raids are being conducted to arrest those involved in the attack.