Three prisoners were shot dead in a clash at Ragauli district jail here on Friday, police said.
One of them died in an exchange of fire with jail officials after he shot dead the other two prisoners, police official SP Tripathi said.
The inmates who died have been identified as Anshu Dixit, Merazuddin alias Meraj Ali and Mukeem Kala, officials said in Lucknow.
"Dixit shot dead Ali and Kala, and held some of the other prisoners at gunpoint and threatened to kill them. He was later killed in the exchange of fire with police," senior police official K Satyanarain said.
Taking note of the incident, Uttar Pradesh Chief Minister Yogi Adityanath said a joint team of Divisional Commissioner D K SIngh and IG, Chitrakoot and DIG (prison) Sanjeev Tripathi will submit a report in this regard.
Asked how weapons reached inside the jail, the Mr Satyanarain said, "The matter is being investigated."
He said all the three inmates were hardcore criminals and had a number of criminal cases pending against them.
However, Ragauli district jailor SP Tripathi earlier in the day said Dixit had snatched the service revolver of a jail staff when the official intervened in the argument between the inmates.
According to sources, Kala, a resident of Shamli, was involved in a number of cases of murder, dacoity and extortion cases, whereas Ali was an active member of gangster-turned-politician Mukhtar Ansari's gang.
Dixit, a resident of Sitapur, was a contract killer who had worked for gangster Munna Bajrangi in the past, they said.
Ali was shifted to Chitrakoot on March 20 from Varanasi jail, while Kala came to the jail on May 7 from Saharanpur.
Dixit had been lodged in the jail since December 8, 2019, officials said.
The situation inside the jail is now under control, they added.