An Indian militia leader with links to Maoist rebels has been arrested over the derailment of a passenger train that killed some 150 people in May.
Bapi Mahato was captured as police stormed a guest-house early on Sunday.
A West Bengal police chief said Bapi Mahato would appear in court on Monday.
About 100 other people were also injured when the express was derailed in West Bengal and some carriages fell into the path of a freight train.
Police accused Maoist rebels of sabotaging the track, but Maoists denied the charge. The rebels say they are fighting for the rights of indigenous tribespeople and the rural poor, who they say have been neglected by governments for decades.
Prime Minister Manmohan Singh has described the Maoist insurgency - which affects parts of several central and eastern states - as India's biggest internal security challenge.
Bapi Mahato is a top leader of the People's Committee against Police Atrocities - a militia said to be close to Maoist rebels.
The BBC's Subir Bhaumik says the militia is active in the Junglemahal region of West Bengal which borders Jharkhand state.
Bapi Mahato was arrested in the town of Jamshedpur, in Jharkand. A relative accused of giving him shelter, named as Bimal Mahato, was also arrested.
"We were sure of Bapi's location but Jharkhand police helped us in pinning him down. They provided us a huge back-up force with which we encircled the guest house where Mahato was staying ," the officer who led the West Bengal police team, Wakkhar Raja, said.
Bapi Mahato has been named by Indian federal investigators as the mastermind behind the sabotage that led to the derailment.
A reward was offered for information leading to his arrest and that of two close associates - Umakanta Mahato and Asit Mahato, who remain at large.
Umakanta Mahato was arrested in June 2009 and charged with sedition and waging war against the state. But he was then released on bail in December and police did not contest this, according to court records.