Khalistan Liberation Force (KLF) chief and 2016 Nabha jailbreak accused Harminder Singh Mintoo died on Wednesday of "cardiac arrest", police said.
Police sources said Mintoo complained of uneasiness after a court hearing through videoconferencing in the Patiala Central jail, around 60 km from here.
Mintoo was examined by a prison doctor and later referred to a government hospital where he was declared dead.
A probe has been ordered into his death.
Mintoo was among six prisoners who made a sensational jailbreak from the maximum security prison in Nabha town in November 2016. He was re-arrested from Delhi's Nizamuddin railway station within a few hours.
Mintoo was earlier arrested from New Delhi's Indira Gandhi International Airport in November 2014 after he returned from abroad. He was accused of involvement in various terror cases, including an attack on Dera Sacha Sauda chief Gurmeet Ram Rahim Singh.
He was accused of hatching a conspiracy, at the behest of Pakistan's Inter-Services Intelligence, to revive terrorism in Punjab.
Mintoo was hiding in Thailand and Malaysia.
--IANS
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