Toronto Police's homicide unit is investigating after two people died from "party drugs" believed to be ingested at the VELD Music Festival in Toronto Sunday night.

At a news conference on Monday evening, police said the victims, a 20-year-old female and a 22-year-old male, were among 15 people who were hospitalized after ingesting substances at the event in Downsview Park.

Investigators say they believe the drugs were purchased at the festival and would like any concert goers in possession of substances to come forward.

Police say they are not sure what particular drug led to the fatalities.

“People are dying because they are ingesting this drug. We want to find that drug. We want to find out exactly what it is," said Deputy Chief Mark Saunders.

"Medical staff have indicated that there is a possibility that it could be poison and we need to determine exactly what we are looking at so that we can make that proper analysis and we can take this investigation through the proper course that it needs to go through."

The Special Investigations Unit has also become involved in the investigation.

According to the SIU, the male victim died after being restrained by police.

A police officer was on duty at the music festival early Monday morning when he observed two people carrying the man.

The officer requested an ambulance and while paramedics tended to the patient, he became violent and had to be handcuffed, the SIU said in a release.

The man’s handcuffs were then removed while he was being treated in the ambulance, but he was pronounced dead while on the way to the hospital.

The SIU is an arm’s length agency called in to investigate whenever officers are involved in a death or serious injury.