And then, on Sunday, her body was found on a beach.
Now, there is weeping in the capital and candles in the snow, as a country that has gone full years without murders asks who, why, how?
“If you are not familiar with the family, you know someone who is,” the columnist, Soley Bjork Gudmundsdottir, wrote. “This whole affair feels personal.”
It has felt that way for nearly two weeks, since security cameras along a downtown street in Reykjavik captured Brjansdottir's last known footsteps.
Brjansdottir zagged down a sidewalk in the predawn hours of Jan. 14 — she was just past a breakup, her parents said, and leaving a club after an annual indie band festival.
A kebab in her hand, swaying, she nearly careened into two people, who barely took notice.
She continued down an empty block, auburn hair shimmying on her shoulders, past a storefront's flashing red light.
Then out of the frame. Then nothing.
Within hours of Brjansdottir's disappearance, people began to worry.
They traced her cellphone to a town 20 minutes down the coast from Reykjavik, where someone had turned it off, according to the Monitor.
There, they found her shoes in the port.
A coast guard helicopter began to search town and countryside.
They were after the Polar Nanoq, a ship that had set sail from the same port where Brjansdottir's shoes were found, on the same day she went missing.
Two sailors on the trawler had rented a car on the night of her disappearance, police said in the Monitor. She had walked right past the car after leaving the club.
Both men were from Greenland, just across an Arctic sea. As news of their arrest and interrogations spread, Iceland President Gudni Johannesson had to warn his citizens against becoming prejudiced against their small neighbor.
Iceland prepared for the worst.
It came Sunday, when a coast guard helicopter flying over a rocky peninsula spotted something near a lighthouse. A body.
“The police believe Birna was murdered in a rental car,” the Monitor reported afterward.
On that long, cold night, candles glowed on two sides of a great water — in Iceland's capital and outside its consulate in Nuuk, Greenland, the red candles buried in snow.
And so a country stopped searching for one of its own and began searching for answers.
It has found none so far. But on Monday, the Monitor reported, Icelandic police said they were looking into a possible link between Brjansdottir's death and that of a 17-year-old who disappeared from a quiet town in Denmark last summer and turned up dead in a lake.
Already, those safe northern waters seem colder.
An earlier version of this story incorrectly identified the waters between Iceland and Greenland as the Norwegian Sea.