A suspected Islamic State operative was killed in an anti-terror operation here early on Wednesday and hours later, a National Investigative Agency (NIA) team arrived to take the probe forward.

The NIA will look into the items recovered from slain Saifullah's hideout in Haji colony in Thakurganj area, where he was killed by the Uttar Pradesh Anti Terrorism Squad in a 11-hour-long gunfight.

According to police, six pistols of different make, two wireless sets, alarm clocks, steel pipes, an IS flag, two laptops, videos of how to make bombs, four knives, two passports and 600-plus live cartridges were recovered from the site.

"We tried our best to capture him alive but were left with no other option," Inspector General (Special Task Force) Aseem Arun told the media while narrating the details of the incident.

Intelligence agencies from two other states had tipped off the police of the likely presence of terror suspects in Lucknow, Kanpur and Etawah after an explosion took place in a train in Madhya Pradesh on Tuesday.

The gun battle broke out around 4 p.m. on Tuesday in Lucknow and continued for over 11 hours, concluding early on Wednesday. 

Two persons were arrested from Lakhna in Etawah, three from Kanpur and one was holed up in Lucknow, Arun said.

Saifullah opened fire when the police reached the hideout to nab him.

Faisal, a suspect, was arrested from Tiwaripur area in Kanpur and his elder brother Imran was arrested from Unnao. They are currently being interrogated by the ATS.

Two others detained by the ATS in Kanpur were set free by a mob.