Customers were shocked Tuesday as they walked up to the Nook, a Highland Park burger joint, and yanked on a door that wouldn't budge.

An early morning blaze charred the St. Paul institution, but that was nearly impossible to discern from the exterior's relative normalcy.

"I love this place," said Greg Lunak, a financial adviser who arrived for a lunch meeting there Tuesday. "It tends to attract an interesting mix of people -- college students, business people, families."

The fire began sometime after 3 a.m. No one was in the business at the time, but tenants who lived upstairs smelled smoke and called 911, said co-owner Michael Runyon.

Firefighters circled the building at 492 Hamline Av. S. but didn't see any flames, he said. They noticed smoke, so one of them put a hand to the front window and felt an unusual amount of heat.

Authorities said the fire spread fast, and snow from the weekend made it difficult for firefighters to put enough water on the flames. Crews had to dig through the snow to find hydrants for their hoses.

Runyon and co-owner Ted Casper arrived about 4 a.m. and were allowed inside a few hours later.

"We walked through the doors ... and it was a nightmare," Runyon said. "It was done."

While the specific cause hasn't been determined, Casper said he suspects "some kind of cooler malfunction" in the kitchen, which suffered most of the damage.

Mariah Howe traveled an hour from Owatonna with her mother and brother to lunch at the Nook on Tuesday in celebration of her 17th birthday. Her older sister, a local, recommended it, saying it had "the best burgers around and great atmosphere." The three were shocked to find it burned.

"It's sad," Howe said. "I wish I could have experienced eating here."

Childhood friends Casper and Runyon, both 30, have owned the Nook for about 10 years, along with the Ran-Ham Bowling Center next door. (It wasn't damaged, but smelled strongly of smoke.) Their fathers, best friends Tom Casper and Pete Runyon, first bought the Nook, which their sons ran before taking over ownership.

Ted Casper and Michael Runyon initially did everything as their mothers waited tables. The business quickly grew, and now employs 25 people.

"I remember a day they made only $87," Pete Runyon recalled Tuesday. "We looked at them and said, 'This just isn't going to happen.' I'm proud of the two of them. They've done a heck of a job."

The two also own Shamrock's, an Irish restaurant on W. 7th Street.

They are intent on rebuilding at the location where the Nook has stood in some form since at least 1938. Twins star Joe Mauer is a customer, and has a burger named after him. He attended high school at Cretin-Derham Hall across the street.

Behind the building, students have etched their names and class year across the white bricks, nearly reaching the top of the building. "Kathy 76," one etching read.

Casper estimated that it will be a month or two before the restaurant can reopen.

The Nook was named "Best Neighborhood Spot" in 2009 by Minnesota Monthly, and City Pages in 2008 declared it the winner in the "Battle of the Burgers" and for having the "Happiest Hour." It also was featured on the Food Network show, "Diners, Drive-ins and Dives."

In an unrelated overnight blaze, firefighters from St. Paul, Minneapolis and Maplewood battled a fire at Advanced Furniture and Leather Repair at 822 N. Prior Av., in the Midway area.

Firefighters were called to that scene at about 3:30 a.m. There is no word on how that fire started.

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