Deepak Kumar, a constable with Patna police, was arrested on Thursday for his alleged involvement in the kidnapping of the owner of a tour and travel agency, Sunil Kumar Upadhyay.

His three other associates were also arrested.

Making the startling disclosure about police-criminal nexus, Patna city superintendent of police (west) Rajiv Mishra said the constable, posted at the Secretariat police station in the capital, was on the radar of the special investigation team (SIT) for the past three days.

On Wednesday, when Deepak's role in the kidnapping of Upadhyay was confirmed, he was summoned at the police station concerned for questioning and subsequently taken into custody.

"The police have enough evidence to proof Deepak's role in the crime," the SP (west) told The Telegraph.

A resident of Harishankar Nagar near Beur in Patna, Upadhyay was kidnapped from Sipara locality of the capital on June 6.

The kidnappers had allegedly demanded Rs 5 lakh as ransom for his safe release. However, the victim was released the next day under mounting police pressure.

During the investigation, the role of Deepak came to the fore. Apart from him, the names of his three associates - Shyam Yadav, Amit Prakash and Sanjit Sao - also cropped up and they all were apprehended. While Shyam was arrested from Dighwara in Saran, Amit Prakash and Ranjit Sao were nabbed from Yarpur and Saristabad in Patna.

Sources said Deepak was posted at the Secretariat police station for more than a year and he took part in every raid conducted in the areas falling under its jurisdiction. "His links with the members of the gang involved in road robbery during night are also being ascertained," an investigating officer said.

The SP (west) Mishra said the constable has already been put under suspension and departmental proceedings have been initiated against him.

"The proposal for the dismissal of his service has also been submitted to the authority concerned," he said, adding that the constable had earned a bad name for the state police force.

Sources said initially the deputy superintendent of police of Phulwari, Imtiyaz Ahmed, received a tip off about the constable's involvement in the tour and travel agency's owner's kidnapping for ransom.

Subsequently, he informed the senior police officers, who ordered a detailed probe into his role.

Deepak was part of the "quick mobile force", which was created to keep a close surveillance on the movement of anti-socials in lanes and bylanes of the capital.

He used to operate from his Gardanibagh rented accommodation, where people with shady characters were often seen.

The family members of the arrested cop met the SP (west) and pleaded his innocence. Deepak is said to be a resident of Rohtas district in south and central Bihar.

The Patna police had busted a gang of kidnappers, which was operating under the garb of Special Task Force (STF) commandos.

Five members of the gang - Munna Singh, Vikas Chandra Srivastava, Shambhu Kumar, Suman Kumar and Sonu Kumar - were arrested for their involvement in the kidnapping a medicine dealer Amresh Kumar Singh and his son Rahul from Pirbahore area in Patna in February 2013.