An unusual deep cut on the thumb of an acquaintance of an 18-year-old youth found murdered in a Kankurgachhi alley on Thursday has helped sleuths identify the killer, officers involved in the probe said.

The 17-year-old youth, who had been an acquaintance of Jogender Das, the victim, has allegedly confessed that he killed him by smashing his head and then slitting his throat with a steel strip.

The suspect - Metro is withholding his name as he is a minor - is said to have told the police that he had attacked Jogender, also known in the locality as Piku, in a fit of rage after he abused him.

The police said they were alerted about the teenager's possible role in the murder after seeing a deep cut on the thumb of his right hand.

"We suspect the injury was caused while slashing Jogender. He broke down during interrogation and admitted to the crime," said an investigating officer.

"The suspect said Jogender abused him for no reason whenever they would meet. The same happened on Tuesday night when the two met in a desolate alley in Kankurgachhi. The suspect lost his cool and hit Jogender with a brick. When the victim fell, the accused slit his throat with a steel strip lying nearby," the officer said.

The police claimed to have found the brick - stained with blood - but the steel strip is missing. "The youth said he had thrown away the strip under the influence of drugs and could not recall the location," the officer said.

Jogender's decomposed body was found beside a garbage vat off UC Banerjee Road, behind the Kankurgachhi Pantaloons, on Thursday morning.

The police said Jogender lived in a slum near the Calcutta Municipal Corporation quarters, around 500m from the spot where his body was found.

Residents searched the area and spotted the body after being alerted by a strong stench.

While probing the case, the police came to know that Jogender was a habitual drinker and a regular at some of the drug dens in the area. Since Thursday, the police started questioning some youths who were often seen drinking and sniffing drugs with him.

"While inspecting the spot where the body was found we came to realise that Jogender had been involved in a scuffle before he was killed. A resident drew our attention to the suspect, saying he had seen the teenager wrap a bandage around a finger late on Tuesday. We immediately called him. During interrogation he broke down and confessed to the crime," the officer said.