A suspected IS operative was killed here in the wee hours of Wednesday after a 11-hour-long anti-terror operation conducted by the Uttar Pradesh Police's Anti-Terror Squad (ATS) commandos.
Many efforts to persuade the suspect, who was holed up in a house in Haji colony of Thakurganj area, to surrender went in vain, an official said.
After the initial exchange for fire, the ATS commandos lobbed chilly bombs inside the house to incapacitate him, but to no avail.
Police teams bored a hole in the roof and blew up the back wall of the building to enter the house.
Senior police officials were camping at the site to oversee the operation, which was also being monitored by the Union Home Ministry.
Initially, it was suspected that there were two IS operatives inside the house but that turned out to be a false alarm as police found the body of only one. One pistol and a revolver was found in person of Saifullah, the suspect.
Earlier, the ATS, based on "concrete" information from intelligence, had gone to apprehend Saifullah, suspected of involvement in a train blast in Madhya Pradesh, when he suddenly opened fire on the team from the house.
An official at the site had said that they were trying to ensure that he is arrested alive and there is no collateral damage as this is a residential area.
"We got a lead on these operatives from outside the state and we are on the trail," said Additional Director General (Law and Order) Daljeet Chowdhary.
At least eight persons were injured in an explosion in a passenger train, going to Ujjain from Bhopal, in Madhya Pradesh's Shajapur district on Tuesday, authorities said.
Sources said the house belonged to Badshah Khan, resident of Malihabad, and the accused had been living there with two others for the past two months. They posed as students and did not mingle with the neighbours, an ATS official told IANS and informed that efforts were now underway to nab the two accomplices who were at large.
Police picked up two suspects from Lakhna in Etawah and three in Kanpur. Following the Lucknow gunfight, an alert has been sounded at the Lucknow International Airport and at prominent railway stations across the state.
Police and intelligence officials have been directed to step up vigil and keep a close watch on busy thoroughfares, malls and markets.