But although officers concluded that the youths were behaving suspiciously and passed on details of their red Y-registration Vauxhall Astra over the police radio, the car was not stopped until more than a week later, the inquiry heard.
It was later established that the car's occupants on the night that Stephen, 18, was stabbed to death at a bus stop in Eltham, south-east London, included Daniel Copley and Jason Goatley, who had been convicted in 1991 of offences connected with the murder of Rolan Adams.
Rolan, a black 15-year-old, died of stab wounds after he and his brother, Nathan, 14, were attacked by a gang of 15 white youths while on their way home from a youth club in Thamesmead, south-east London. Another occupant of the car, the inquiry was told, was Kieran Hyland, a "leading light" in a racist organisation called Nazi Turn-Out, known as NTO, which was active in nearby Eltham, where Stephen was killed.
Sergeant Andrew Hodges, a member of a Territorial Support Group unit, said he saw the Astra - containing five white youths who were "laughing and joking" - drive past the scene of Stephen's murder just after he had helped to cordon it off. A few minutes later, the car drove past again in the opposite direction. Under cross-examination by Michael Mansfield QC, counsel for the Lawrence family, Sgt Hodges agreed that no attempt was made to flag it down.
Five white youths were eventually charged with Stephen's murder, but the case did not reach trial and a private prosecution by his family was unsuccessful. The inquiry is examining issues arising from his death.
Sgt Nigel Clement, who was with Sgt Hodges and radioed the Astra's details, stopped the car - then containing just Daniel Copley and Kieran Hyland - when he spotted it in the area eight days later. But he told Ian McDonald QC, counsel for Duwayne Brooks, a friend of Stephen's who was with him when he was attacked, that he knew nothing of the two youths' backgrounds, nor of that of Jason Goatley, until yesterday. "What you are telling me now is the first indication that I am aware of," he said.
John Sentamu, Bishop of Stepney and a member of the inquiry panel, asked Sgt Clement why other police vehicles at the murder scene did not set off in pursuit of the Astra. "Why did it not happen?" he asked. "Why did someone not say 'We've seen five youths, let's get after them'?"
Sgt Clement replied: "It's hard to say. In an ideal world, that vehicle would have been stopped on the night, and it would have been a great help to us."
Inspector Steven Groves, the senior officer first in charge of the murder scene, told the inquiry that he knew nothing about the Astra. "I don't recall the car, or being told about it," he said.
The inquiry continues today.