Police used dogs, a boat and a helicopter to search for Brandon Crisp, a Barrie, Ont. teen who hasn't been seen since Thanksgiving Monday and who hasn't tried to contact friends.

Now police are starting to worry about whether Brandon, a fan of outdoors wilderness TV shows, can adequately shelter himself against the night-time cold. No break-ins have been reported at area cabins.

"We've been monitoring the weather and looking at weather patterns since the time he left home," Sgt. Barrie Goodbrand of the Barrie Police told CTV Toronto on Thursday. "There have been a number of nights that have been in the -4, -5 range. We had snow on the ground two nights ago. So we're concerned about Brandon's safety."

More than 800 tips have poured in from the public. Police say they will continue the search through the weekend. They are focusing on the areas around Shanty and Kempenfelt Bays on Lake Simcoe. Hundreds of volunteers are helping out.

At St. Joseph's High School in Barrie, prayers were said at a vigil. Inside the school's packed, silent auditorium, candles flickered beside photographs of Brandon placed on the stage.

Brandon's friends at the high school have also started a graffiti wall where everyone has written a message to him.

"I just hope we see Brandon home today, with all the support and all the love and all the prayers," said Angelika Crisp, Brandon's mother. She attended the vigil, along with her huband.

On Oct. 13, Brandon, 15, left his house following an argument with his parents after they confiscated his Xbox.

Later that day, after his parents reported him missing, police found Brandon's bicycle in a ditch northeast of Barrie.

On Wednesday, police said a woman came forward saying she spoke to Brandon the day he disappeared after noticing he was having trouble with his bike. Brandon had been walking towards Orillia, which is northeast of Barrie.

Goodbrand said the information received from the woman was helpful because police had wondered whether the bike could have been put in the ditch by somebody else.

Is Brandon still in Ontario?

Goodbrand also said police are considering the possibility that Brandon could be out of the province.

"That's why we're looking at his computer usage, some of the users that he's had contact with, see where they reside, look at their IP address," he said.

"...It's important for us to push this story nationally because the more people across this country that know Brandon's face and his story it's more likely that they'll see him and be able to lead us in his direction."

Brandon's parents fear their son may have become mixed up with people he met while playing video games online.

He had become addicted to the game "Call of Duty 4: Modern Warfare," they told CTV in an interview Wednesday.

Brandon had skipped school on the Thursday prior to Thanksgiving weekend, causing his parents to revoke his Xbox privileges -- a disciplinary measure they had used, his father said.

He is described as male, white, 100 pounds, 5'2", with short sandy blond hair.

When Brandon was last seen, he was wearing a yellow and grey jacket, a grey American Eagle-brand hoodie, blue jeans, white runners with a camouflage stripe on the sides. He may also be carrying a yellow and grey back pack.

Anyone with information is asked to contact Barrie police at 705-725-7025.

Metroland Media Group Inc., which publishes the Barrie Advance where Brandon's mother works, is offering a $10,000 reward for information that helps police find the boy.

"We're hoping that anyone, even with the tiniest bit of information about seeing Brandon in the time since he's been missing, will be motivated to come forward," said the newspaper's Charmaine Nolan.

With reports by CTV Toronto's John Musselman and Ken Regular