The tragedy occurred at the Rajmahal Opencast Project of Eastern Coalfields Limited (ECL).
Jharkhand Director General of Police D.K. Pandey said more than 20 workers were still feared trapped inside the collapsed mine.
According to a statement by the Coal Ministry, the incident occurred around 7.30 p.m. on Thursday.
Besides ordering a probe, the ECL has announced an ex-gratia compensation of Rs 5 lakh each to the families of those killed, in addition to the amount to be paid under the Workmen’s Compensation Act.
“An inquiry has been ordered in the incident by the Director General of Mines Safety and a High Level Committee of Experts has been constituted by Coal India Limited to investigate the causes of the accident,” it said.
“Prima facie, it is observed that the incident is unprecedented, since an area of 300 metres length by 110 metres width solid floor of the over burden dump area slid down by about 35 metres involving around 9.5 million cubic metres of earth material,” it said.
“This could be due to failure of the bench edge along the hidden fault line/slip,” it said.
A heap of mud caved in at the entry point of Lalmatia mines of ECL. At that time more than 70 vehicles were inside open cast mine the police said.
Prime Minister Narendra Modi expressed grief on the tragedy and spoke to Chief Minister Raghubar Das to take stock of the situation.
Modi said the Jharkhand government and Union Power and Coal Minister Piyush Goyal were working to restore normalcy.
The National Disaster Response Force was carrying out rescue operations monitored and supervised by senior ECL and state government officials.
The ECL has also set up a control room, headed by General Manager (Mining) R.R. Amitabh, to monitor the situation.