Police are offering a £20,000 reward to anyone who can provide information following the unprovoked murder of a young man in west London.
The 18-year-old victim, since named as Jason Isaacs, died after being attacked "for no reason" by a gang of moped riders at Newnham Close in Northolt on 18 November.
The incident occurred as he was walking down the street with friends to go to a party. Isaacs and his friends then spotted two mopeds, both with pillion passengers wearing balaclavas.
Isaacs was set upon and attacked after he became separated from his friends when they attempted to run away.
He was taken to hospital suffering from critical injuries but died on 21 November. A post-mortem examination is due to be held on 24 November.
A murder investigation has been launched and police are now offering a serious crime reward of £20,000 for anyone with information that leads to the identification, arrest and prosecution of the suspects.
A 17-year-old boy previously arrested on 20 November in connection with the murder has been bailed to return on a date in mid-December.
Detective Chief Inspector Noel McHugh, leading the investigation, said: "On the evening of his murder, Jason was planning to go to a party with four friends. They had bought alcohol from an off-licence in Oldfield Circus, and then walked the short distance down Orchard Close and Newnham Gardens into Newnham Close where they became aware of two mopeds, both with pillion passengers wearing balaclavas.
"Jason and his friends ran and the mopeds gave chase. It was at this point that Jason became separated from his friends, and he was brutally attacked. After the suspects left the scene, Jason's friends went back to Newnham Close to find him mortally wounded.
"This appears to have been an unprovoked, pre-planned attack. Jason, who was unarmed and not looking for trouble, was stabbed multiple times with a large knife.
"Jason was a popular lad who has had his life brutally ended for no reason, leaving his family and friends devastated. We will do everything we can to track down his killers who acted in a truly horrific manner."
In a statement, Isaacs's family said they are "devastated" that the 18-year-old died in such "senseless circumstances".
They added: "Jason was a kind and loving son and brother who adored his music, fashion and his beloved QPR football team. He was working hard as an apprentice carpenter, loved socialising and was a very popular boy.
"We feel that knife crime in this area, and in London, is out of control and it must be stopped before another young life is taken, and another family has to suffer what we are suffering now. We need to educate young people early on around the dangers of carrying knives in public places.
"We are pleading with the public to come forward and assist police in their enquiries to find out who murdered our son as we are currently living in a nightmare; our family is incomplete without Jason and will never be the same again."
Anyone with information concerning this attack is asked to call police on 020 8785 8099 or via 101.
You can also call Crimestoppers anonymously on 0800 555 111. You can also tweet information to police via @MetCC.