Toronto police believe the person who gunned down a teenager in a Regent Park highrise last month was still in the building when officers arrived immediately after the shooting.

Police were called to the building at 605 Whiteside Place after gunshots were reported on the afternoon of Jan. 18.

Inside a stairwell on the 13th floor, they found Tyson Bailey suffering from gunshot wounds. The 15-year-old was rushed to hospital, but died of his injuries.

On Thursday, Det.-Sgt. Justin Vander Heyden said police were returning to the building to talk to residents.

Vander Heyden said investigators have analyzed video recorded at the building and determined that the shooter remained in the building as police sealed off the exits the day that Tyson was shot.

"I can tell you that 51 Division responded so quickly to this phone call that I'm confident that the shooter did not make it out of this building before the police arrived," he told reporters.

That's why police are asking residents "a series of specific questions to try to find out if anybody saw or heard anything that might let us know where the shooter went after shooting Tyson."

Police have not yet identified a specific suspect, but Vander Heyden said they believe the shooter was present when officers arrived on Jan. 18 and likely still there when homicide investigators canvassed the building that evening.

"Whether they were being harboured in an apartment by somebody here, or whether they actually live here is yet to be determined," he said.

Vander Heyden said the people in the building can help solve Tyson's killing.

"Criminal investigations are all about connections and I've established the connection between Tyson and this building," Vander Heyden said.

"I'm sure of why Tyson was here, I'm sure when he came here and I'm sure why he came here. What I need to know is the connection between the shooter and this building and I’m confident there is a connection and the answer lies in that building."

Vander Heyden said an examination of the phone records of Tyson and other individuals has allowed police to determine why Tyson was at the building on the day he was shot.

Video released

The public is being asked to look at security video clips that police have released of two males seen following Tyson into the building on the day he was shot.

Police have spoken to one of the males in the video, whom has been identified as a person of interest and is someone that Tyson knew and was friendly with.

"This person knows Tyson, knows his family, they are friends. So that's why he reaches that [person of interest] threshold," Vander Heyden.

But investigators want to identify the second male seen walking with Tyson and the other individual.

This third unidentified person is also a person of interest, Vander Heyden said.

"He is a person of interest because we have no idea what his relationship may or may not be to Tyson or to the first person of interest."

Vander Heyden said this unidentified person is a black male and was wearing black clothing with a hood pulled up that day.

Tyson is one of three male teenagers shot to death in Toronto since the start of the year.

Just a few weeks after Tyson died, another 15-year-old male, St. Aubyn Rodney, was fatally shot in a Toronto Community Housing building located at 40 Turf Grassway, in the Jane Street and Finch Avenue area.

Police have charged a 17-year-old with manslaughter in connection with St. Aubyn’s death.

And last Sunday evening, 15-year-old Jarvis Montaque was killed by an assailant who walked up and shot him at close range on Jamestown Crescent home in Rexdale.

Police have said they do not know why Montaque was targeted.

In another shooting, nine-year-old Kesean Williams was shot and killed as he sat in his home in Brampton, Ont.  No arrest has been made in that homicide.