By the time 18-year-old Emma van Nostrand collapsed near Lakeshore Boulevard West and Roncesvalles Sunday morning, she had already been running for three and a half hours, and was just three kilometres from finishing the race – her first ever full marathon.

The Grade 12 student from Cape Breton Island had travelled to Toronto a few days earlier with her parents and was excited but nervous about attempting the marathon. "Just an average Sunday morning, any suggestions on what we should do? #kidding #freakingout," she tweeted the morning of the race.

Six hours later, she collapsed on the race course after what police called a "medical emergency," and was pronounced dead in hospital. The exact cause of her death is not yet known.

Story continues below advertisement

Andrew McKay was watching the race with his family when he noticed a group of runners bent over, some of them shouting. He ran over and saw a young woman lying on the ground wearing bib number 375 with the name "Emma" written under it.

"She was completely out of it with her eyes open and her mouth open," he said. "People were shouting Emma, Emma, Emma," he said, but she didn't respond. He used his cellphone to call 911 while another bystander began performing CPR.

Those who knew the teen describe her as an excellent student and athlete, excited to graduate high school in just a few months and study science at Dalhousie University.

Ms. van Nostrand belonged to a family of runners, said Lauchie McKinnon, a member of the Cape Breton Road Runners Club who is a family friend. Her parents Steve and Katherine are both active runners, often bringing Emma and her siblings along on road trips for races. Though this was Emma's first full marathon, she had completed several half-marathons before in the past.

"She was a part of our running community," Mr. McKinnon said. "She was well-liked. It's a shock."

On top of long-distance running, Ms. Van Norstrand also played basketball and soccer.

Adrian Nixon was Ms. van Nostrand's soccer coach three years ago, and described her as "a prince of a person to know" who was always volunteering to help out younger players. "She had a super personality to meet people and make people feel very comfortable, and an easy person to talk with for her age," he said. "She was just an all-around super person in so many ways," he said.

Story continues below advertisement

He said Ms. van Nostrand didn't have any health problems that he knew of.

Ms. van Nostrand's principal Joe Chisholm at Riverview Rural High School near Sydney, N.S., called Ms. van Nostrand a "great student" who loved to travel. Earlier this year, she participated in a student exchange program in France to improve her French, Mr. Chisholm said.

"Even though she had to do correspondence to get through the first semester, she still attained her honours standing," Mr. Chisholm said.

Photos posted on her Facebook page show a smiling Ms. van Nostrand in photographs of that trip, posing in front of Disneyland Paris and the Eiffel Tower.

Ms. van Nostrand, "was a very popular girl," Mr. Chisholm added. "She had a lot of friends in the school, and she will be sorely missed."

Another runner, a man, also collapsed during the Toronto race, but bystanders and paramedics were able to resuscitate him.

Story continues below advertisement

Before Sunday's race, runners observed a moment of silence in honour of the victims of the Boston Marathon bombings last month. Mr. McKinnon said that the van Nostrand family was in Masachusetts during the bombings, cheering on Steve as he ran in the race.

According to an interview Mr. van Nostrand gave to the Cape Breton Post at the time , the explosions happened just as he returned to the hotel a block away from the finish line. "I called my kids because they'd been near the finish line watching," he said. "I was hoping they were in the hotel room and luckily, they were."

Mr. McKay, a father himself, said that he wants Ms. Van Nostrand's parents, who he's never met, to know that their daughter was not alone in her final moments. "I know her dad was running the half-marathon and waiting for her at the finish line. Her mom was running and probably didn't know what happened," he said.

"I saw it happen. Three kilometres from the finish line – you're starting to think 'I'm near the end, I'm almost there,'" he said. "But still, a lot of people stopped running at that point just to make sure she was being taken care of."

With a report from The Canadian Press