The death of a Brampton teen who was killed after chasing down his lost cellphone in London, Ont., has prompted police to warn people of the dangers of using tracking apps.

Jeremy Cook, 18, lost his cellphone in a taxi, and traced it to a parking lot on Highbury Avenue early Sunday morning, according to London Police. Police say Mr. Cook approached a car with three people in it. One of the people got out, and when the car started to pull away, Mr. Cook grabbed the driver's side door.

Shots were fired and Mr. Cook was hit multiple times, police said. He was found behind a nearby plaza and pronounced dead at the scene. Police found the abandoned vehicle and cellphone nearby.

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Constable Victor Kwong of the Toronto Police Service said the case highlighted why people who are tracking stolen cellphones should not confront thieves.

"The major danger is that you have someone who has committed a crime, and when confronted … you don't know what their reactions might be," Constable Kwong said.

Instead, he recommended people first wipe the data off their phone remotely, if they're using a tracking app. After that, they should report the theft to police, he said. If it's not a stolen phone but rather a case of lost and found, Constable Kwong said an exchange is all right, but should happen in a public place.

"We have no problem with people saying 'Meet up in front of a police station' to do these things. That usually will deter a lot of criminality," he said.

On Tuesday, flower bouquets, a stuffed toy cat and rabbit, and a small, white figurine of the Virgin Mary lay against the wood fence lining the alleyway behind the Shoppers Drug Mart where Mr. Cook was found. Another small pot of yellow flowers bore a note that said, "For Jeremy. A young friend gone too soon."

Police arrested three males who were seen leaving the area, but released them after they were found to not have been a part of the incident. They are now looking for three suspects between 18 and 21 years old.

The first is described as wearing a white shirt with a black design. The second suspect is described as having very short hair, wearing a black jacket or shirt and a fitted hat, and the third suspect is of slim build, wearing a blue shirt and a black hat.

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Mr. Cook graduated from Notre Dame Catholic secondary school in Brampton in 2014. He was also a member of Urban Krav Maga in London, Ont. His instructor, Dan Morgan, said he was devastated when he first received a text from one of his students telling him Mr. Cook had been shot.

"Whenever you hear someone that you know died, doors open up inside you, and there's this bottomless pit of feelings, like a nothingness," he told The Globe in an interview.

Though Mr. Morgan teaches classes that prepare students for moments of self-defence against weapons, what Mr. Cook faced was extreme, he said.

"You've got to understand that knives and guns end lives. Especially guns. That's what they're designed for," Mr. Morgan said.

Mike White, who lives on Wyndham Crescent near Baker and Huron Streets, said police officers knocked on his door at about 5:30 a.m. and searched backyards and rooftops for hours, telling homeowners they were looking for a gun.

"It's senseless," he said of the shooting.

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Justin Yong, who attends school in the area, stood outside a nearby McDonald's Tuesday afternoon, discussing the shooting with some friends. They agreed the reason for the crime was shocking.

"How are you supposed to sleep when someone's going to get shot over a cellphone?" Mr. Yong said.

His friends Andrew Emes and Brandon McKay added they wouldn't approach someone while tracking down a lost phone.

Mr. McKay said, "My phone isn't worth my life."

With a report from the Canadian Press

With a report from The Canadian Press