Four people were shot inside a highrise elevator in Whitby on Thursday, leaving one teenager in critical condition as police search for suspects.

The shooting follows a month of gun violence that has left five young people under the age of 19 dead across the GTA and investigators painstakingly searching for multiple suspects.

According to a Durham Regional Police source, the four males entered the elevator at 101 White Oaks Ct. off Dundas St. W. before 12:30 p.m. Thursday and rode it up to the third floor.

When the doors opened, one or more assailants were waiting and opened fire, the source said.

Managing to get the doors shut, the four males, all suffering gunshot wounds, travelled up to the seventh floor where they left the elevator and called police, the source said — just as many residents were calling to report the gunshots heard throughout the building’s mid-levels.

“I heard the shots but I didn’t know what the hell it was,” said resident Scott Dunn, who lives on the fourth floor. “It sounded like pops, like someone took a string of firecrackers and threw it down.”

A 19-year-old victim was taken by ORNGE helicopter to St. Michael’s Hospital. The other three males received immediate medical attention for non-life-threatening injuries, said Sgt. Nancy van Rooy.

The victims’ names, remaining ages and relationships to each other have not been confirmed. It is not known if any were residents of the building.

On the third floor, police officers guarded the elevator area well into the evening, where yellow forensic markers were spread across the floor.

Wayne Munro, who has lived on the floor for two years, heard the shots from his apartment — “I thought I was hearing people setting off firecrackers” — before he came out into the hall where other residents told him they’d seen three men in hoodies run into the north stairwell.

Munro approached the elevator area, where several bullet casings lay on the floor and he saw bullet holes in the wall and in one of the three elevator doors.

He pocketed some of the casings in his shirt. Realizing his mistake, he went down to make a report to police, who had swarmed the front entrance, and return them.

On the seventh floor, more police officers guarded another crime scene in the hallway where a paramedic’s board stretcher lay on the floor among other debris: empty water bottles and what appeared to be clothing.

One seventh-floor resident, who did not give his name, said he heard shouting from the hallway following the shooting. “A lot of noise and calling for help,” he said, describing blood on the wall outside his unit.

In the first-floor lobby, yet another officer guarded an open elevator taken out of service.

Someone had doused the elevator with bleach after the shooting, a police source said.

Residents leaning out of their balconies to survey the police and media gathered below described hearing the commotion

“It just sounded like . . . chaos,” said a woman who lives on the sixth floor. “It just kept going.”

When the woman opened her apartment door, she said she heard screaming coming from the stairwell.

Several residents said there have been other police calls, including stabbings, at the 20-storey building, whose front lawn hosts a small playground and a daycare on the first floor.

In 2007, a 42-year-old man was stabbed in the lobby after a fight broke out between him and several men.

Police set up a perimeter around the building Thursday afternoon after more than a dozen police vehicles arrived. Tactical and canine units scoured the area for suspects and evidence.

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Homicide investigators and the forensic identification unit conducted a preliminary investigation, as is protocol, said van Rooy. Meanwhile, officers were canvassing residents.

Police also plan to sift through surveillance footage collected from the area, van Rooy said.

A shooting victim walked into a Scarborough hospital on Thursday and police were trying to determine whether that person is connected to the Whitby incident.