A crazed knifeman who stabbed a Polish teenager to death as he tried to defend his girlfriend was at the centre of a police manhunt today.
Investigators believe they know the identity of a black man who murdered Marcin Bilaszewski, 19, outside Finsbury Park Tube station.
The teenager suffered a fatal stab wound as he argued with the man after celebrating the 18th birthday of his girlfriend Ania Betlinska.
Police seized CCTV from a number 254 bus where the man pestered Miss Betlinska before punching her as she got off.
They have also recovered some of the attacker's possessions from the scene and examined records of Oyster cards used to pay for fares on the fateful journey.
Dozens of witnesses, including many from Mr Bilaszewski's group, came forward after the attack at 11.20pm.
They described how the suspect lunged at the victim with a knife after a scuffle before escaping on foot.
Mr Bilaszewski, of Tottenham, north London, was the 11th teenager to be murdered in London this year and the eighth in the past six weeks.
Last Wednesday Nicholas Pearton, 16, was fatally stabbed after being attacked by a mob in Sydenham, south east London.
Mr Bilaszewski is believed to have travelled to Britain five years ago and worked with his father, who lives in the Kingston area, as a builder.
Eyewitness Dagmara Drzymota said the group of up to 25 people had been out celebrating two 18th birthdays.
She said the suspect started shouting abuse at them while they were on the bus and pulled out a knife when Mr Bilaszewski stepped in.
Miss Drzymota said: "He seemed disturbed because of the way he was acting. I think he was on drugs."
Mr Bilaszewski was stabbed as he stood in Seven Sisters Road near the Tube station. He was taken to a north London hospital by ambulance but died an hour later.
Victor Chidi, who works at nearby Rowans Tenpin Bowl, said there was another stabbing in nearby Stroud Green Road some months ago.
A worker at a cab company close to the station said she was "sick" of hearing about acts of violence.
She said: "They should install CCTV cameras to catch these people. It has got to stop."
* Anyone with information should call police on 0300 123 1212 or Crimestoppers on 0800 555 111.