Rajbir Singh, a controversial Delhi Police 'encounter' specialist with over 50 kills to his credit, was shot dead on Monday night in neighbouring Gurgaon allegedly by a property dealer over a dispute on "investments" he had made with the realtor.

The 48-year-old assistant commissioner, whose career had its share of both glory and controversy, was gunned down at the office of the real estate agent Vijay Bhardwaj after an altercation over payment of profit money in a property deal.

Singh was killed by his own gun given to Bhardwaj three days ago, Gurgaon Police Commissioner Mohinder Lal told a press conference.

The controversial police officer, who was involved in the investigations in the terrorist attack on Parliament House and the Red Fort [Images], would have taken precautions had he known that Bharadwaj had enmity with him and had "something in his mind", he said.

"He would not have given his revolver to a person had he known he would kill him," Lal said.

Haryana Director General of Police Ranjiv Dalal said Singh, who enjoyed 'Z' category security, knew Bharadwaj well and was doing "all these shady things."

"So he decided to go alone or otherwise he would have gone with his own people," the DGP said.

"I do not know what business transaction was going on between these two people. That is a matter of further investigation. But the fact is Singh had invested money and the property dealer was not able to pay back and all this led to this bad incident," he said.

"As far as local police tells me, they did not know anything else about Singh's involvement or any other land deals. But we will have to investigate this," the DGP said, adding the probe will be conducted by a special team.

Bhardwaj, who surrendered soon after the incident, "confessed" to the crime, Lal said adding Singh had threatened to kill him if he did not pay up.

Singh, a recipient of President's Police Medal for Gallantry and four other awards, was shot twice from point blank range in Sukhrali village of Gurgaon at around 8 pm.

Dalal said Singh had gone to collect the money, which he had invested with the real estate agent.

He said Singh, who had `Z' category protection because of threat from militants, was not accompanied by his security personnel when he was shot at.

Talking to reporters, Bhardwaj said he owed money to Singh and that the officer had "threatened" to kill him.

"I wanted to commit suicide, but my family found the (suicide) letter and stopped me. Today was payment day and I could not put the money together. He (Singh) said he would kill me," he claimed.

Singh, who had joined Delhi Police as a sub-inspector in 1982, earned the title of 'encounter specialist' as he was involved in 56 shootouts, including the controversial Ansal Plaza operation in 2002 in which questions were raised over the killing of two terrorists.

Singh was transferred from the Crime Branch to Delhi Armed Police after allegations of his involvement in an illegal land deal last year.

There were also allegations of his links with drug traffickers, land mafia and bookies following telephone tappings.

The vigilance probe, however, gave him a clean chit and he returned to head the Special Operations Squad in November 2007.

Delhi Police said it would work closely with Gurgaon Police in the investigation into the killing.

A Delhi Police team, which went to verify details of the incident, identified his body which has been sent to the civic hospital in Gurgoan for post-mortem.