A trio still some way from attaining legal drinking age had gone across the road from 6 Sunny Park last Saturday and bought three bottles of vodka for the get-together where 17-year-old Aabesh Dasgupta was later found fatally injured, police sources said.

Doctors who conducted the post-mortem on Aabesh told police that they had found alcohol in his stomach, a finding that matches the accounts of several teenagers who told Metro that some of them had been drinking before tragedy struck. "I am not comfortable with alcohol but most of the others were drinking," said one girl, a student of Class XII.

Legally, liquor cannot be sold to anyone under 21. Excise rules governing the sale and consumption of liquor prohibits shops and bars from selling anything containing alcohol to minors but do not provide any guidelines for confirming age eligibility.

This loophole - a liquor salesperson is apparently expected to determine age solely on the basis of physical appearance - possibly allowed the three underage boys to buy alcohol, sources said.

The rule-book states: "Licensee (the owner(s) of the shop or bar) shall not sell or deliver liquor (foreign/country) to any person apparent under the age of 21 years."

The operative word in the rule is "apparent". Any store found violating the rule runs the risk of its licence being revoked or being made to pay a fine of Rs 3 lakh for a single offence.

If an underage buyer is caught with liquor bottles, or in a state of intoxication, it is the police's responsibility to take action. But the penalty is a pittance. "We can book the offender under Section 268 of the IPC that deals with 'public nuisance'. The penalty is up to Rs 200," a police officer said.

Aabesh bled to death from a deep cut under his left armpit that his friends said was caused by broken glass after he tripped and fell with a bottle under his arm. This was presumably one of the bottles that he and his friends had bought from a liquor store on Syed Amir Ali Avenue, opposite Birla Mandir.

One of the friends had even posted a picture of himself with the three bottles on Facebook. When Metro went to the store on Monday, the employees at the counter said they couldn't recognise the face from the picture posted on the boy's Facebook wall.

"So many people come to our store. Saturday evenings are usually packed. It is impossible to remember one face," said Soumyajoti Saha, a co-owner of the store.

Asked how his employees determined whether a potential buyer was age-eligible, Saha put it down to perception. "It is impossible to tell somebody's age by looking at the person. There is no mandate to check age proof. If someone seems under 21, we don't sell. But often, you can't tell a 19-year-old from a 21-year-old."

Sometimes, young people remain seated in their cars and send a driver or attendant to buy liquor, said an employee at the store.