Josh Reeson, 15, died in York Hospital on Sunday, September 27 after taking illegal drugs.
Officers say they now know Josh, from York, and a 14-year-old friend left an address on Fulford Road in York at around 3.10am and 3.15am.
From there, they walked down Fulford Road towards town and then double-backed slightly and went down Broadway West.
The two boys then walked back onto Fulford Road, where they met a 14-year-old female friend near to the junction of Maple Grove.
It is at this point - on the junction of Fulfold Road and Maple Grove - that Josh became ill.
A relative of Josh’s friend called the ambulance and stayed with him until it arrived.
Officers are appealing to anyone who thinks they might have seen the teenagers in the area between 3am and 4am, or anyone who was driving in the area at the time and may have dashcam footage to contact North Yorkshire Police.
Anyone who can help should call 101, press option two and ask to speak to CID York, or email [email protected], quoting reference number 12200169915.
Information can also be passed on anonymously via Crimestoppers on 0800 555111.