Indian bomb disposal experts have found traces of a high explosive in a passenger train which was rocked by a blast on Thursday, officials say.
The blast destroyed an unreserved compartment
At least 10 people were killed and more than 50 others injured in the explosion on the Shramjivi Express in the northern state of Uttar Pradesh.
Traces of the high explosive RDX have been found in the compartment where the blast occurred.
The train was travelling from Patna in eastern Bihar state to Delhi.
Indian Railways Minister Laloo Prasad Yadav has ordered an investigation into the explosion, which happened near the town of Jaunpur in Uttar Pradesh.
A senior Jaunpur police official Devraj Nagar told Reuters they were investigating whether the blast was the work of militants.
"The nature of the blasts suggests the use of RDX explosive. A terrorist hand cannot be ruled out," he said.
Mr Yadav told the Press Trust of India that the "condition of 20 (injured passengers) is serious".
The unreserved compartment where the blast took place was packed to capacity, and eyewitnesses said people were blown to pieces or thrown out of the train.
A BBC correspondent in India says police are also investigating the possibility of any connection with an attack on the disputed religious site in the northern town of Ayodhya earlier this month.
The police say similar explosives were used in the Ayodhya attack, which was blamed on Muslim militants.
Mr Yadav had earlier told reporters that it was unclear whether the explosion was caused by a bomb or by the gas cylinder of a cooking stove, sometimes carried by villagers on trains in India.