The appeal was made after officers released without charge two men and a juvenile, who had been arrested for questioning about the murder in the Somerton Road area off the Antrim Road in the north of the city.
One of the men was immediately re-arrested for questioning about an unrelated matter, following a search mounted as part of the murder investigation.
The PSNI said the murder investigation was ongoing and they appealed for any members of the public who were in the area of the Fortwilliam service station or the Somerton Road on Wednesday night and saw anyone or observed any suspicious activity to contact them.
Appealing for help in tracking the killers, Detective Superintendent Colin Sturgeon said: "Thomas was attacked and callously murdered and I`m appealing to the public to help me catch those responsible."
He said it was an horrific attack which had sent shockwaves through the community and devastated the schoolboy`s family and friends.
"Nothing could justify the murder of a 15-year-old schoolboy, those who could commit such a crime have absolutely nothing to offer society."
Appealing to the friends and families of the attackers he added: "If you know or have reason to suspect someone who was involved you need to speak to police immediately.
"I would ask them to consider how they would feel if this happened to one of their family members or friends. Those responsible need to be brought to justice."
PA