The schoolboy knifed to death after his GCSEs had a terrible secret: he was part of a notorious south London gang.

Pictures on the gang's website show Alex Mulumba, 15, masked and crouching with a pump-action shotgun.

Although he had no previous convictions, he was on police bail for charges including robbery, theft and violence.

Alex was stabbed once in the heart on Thursday after getting off a bus with two friends in Lambeth, south London. The trio had been out celebrating the end of their GSCE exams in Brixton.

Three boys, aged 14, 14, and 16, arrested over his murder on Saturday, have today been bailed until July. Two men, aged 18 and 20, are still being questioned by police.

Alex's heartbroken father, Kamondo, 48, said his son was a "good boy" who had "nothing at all to do with knives".

His boy wanted to study electrical engineering at college, added Mr Mulumba, who brought his family from Zaire to London in 1991 to escape the violence back home.

In an attempt to show the danger of carrying knives, the family earlier released a mobile phone image of Alex as he lay dying in St Thomas Hospital.

Police confirmed they were investigating the possibility that the 15-year-old was killed in an attack by a rival gang. Detectives were aware of the website, a Met spokesman said.

The website, which carries DIY terror tips, also pictures Alex (nicknamed "Tiny Alien") shaping his hand like a pistol while another boy holds a shotgun above his head.

Last year two 15-year-olds linked to the gang, Falco Moludi and Ipaon Modeng, have been jailed for more than six years over 35 muggings and other offences. They posted lurid details of their crimes along with rap lyrics on the same website.

Detectives believe Alex and his two friends were "coralled" by about ten youths in Black Prince Road at around 11pm last Thursday.

Some of the teenagers ran into the Ethelred Estate to fetch weapons including hockey sticks and baseball bats. A fight followed, during which Alex's heart was pierced.

His mates flagged down a motorist, who drove the bleeding boy to St Thomas' Hospital. He died shortly after midnight.