Mujahid was being brought back to Lucknow jail after hearing in a Faizabad court this afternoon, Additional Director General of Police (law and order) Arun Kumar told PTI.
The accused fell sick and fainted near Barabanki border and was rushed to district hospital where doctors declared him dead this evening, he said.
He was also an accused in 2007 Gorakhpur blasts.
Meanwhile, the Lucknow jail administration has requested for a magisterial probe into the death.
However, on May 10, a special court in Barabanki rejected the UP government's plea. They were arrested on December 20, 2007 with arms and explosives.
The UP government had requested the court that it wanted to withdraw the case in "public interest and for the sake of communal harmony", but did not defined both the words.