Friðrik Sigurðsson, sometimes written Fridrik Sigurdsson, was born around 1811 in Iceland. Very little is known about his early life, but by the time of the murders, he was around 18 or 19 years old and was living in northern Iceland. Historical accounts suggest that Friðrik may have been romantically involved with Agnes Magnúsdóttir, one of the two maids working at the remote farmstead Illugastaðir, where the murders took place.

Like many young men of the time in rural Iceland, Friðrik would have grown up in a harsh environment, shaped by poverty, isolation, and limited opportunities. The early 19th century in Iceland was marked by social rigidity, economic hardship, and an unforgiving judicial system rooted in Danish colonial rule.

His association with Agnes and involvement in the household at Illugastaðir eventually led him down a fatal path. He was believed to have been jealous or resentful toward Natan Ketilsson, a farmer and herbalist who also had a complicated relationship with Agnes.

The crime that sealed Friðrik Sigurðsson’s fate took place on the night of 13 March 1828, at Illugastaðir, a farm located in Húnavatnssýsla, northern Iceland.

That evening, Friðrik arrived at the farm and conspired with Agnes Magnúsdóttir and Sigríður, another maid. He instructed the two women to hide in the barn while he waited for the two men on the farm, Natan Ketilsson, the owner, and Pétur Jónsson, a visitor, to fall asleep inside the main living area, known as the baðstofa.

Once the men were asleep, Friðrik and Agnes entered the baðstofa, and Friðrik stabbed both Natan and Pétur to death with a knife. After the murders, the bodies were left inside, and the room was set on fire in an attempt to cover up the crime. It is unclear whether Sigríður participated directly in the killings, but it was later revealed that she stole valuables before the fire was lit.

In the early hours of 14 March 1828, Agnes appeared in the nearby town of Stapakoti in Vatnsnes, claiming that Illugastaðir had caught fire accidentally and that both Natan and Pétur had been trapped inside. However, once the fire was extinguished and the bodies were recovered, it became clear the fire had been a cover-up. The men’s bodies showed clear signs of violent death, including stab wounds.

An investigation followed, leading to the arrest of Friðrik, Agnes, and Sigríður. During the trial, Friðrik and Agnes were found guilty of murder, while Sigríður’s involvement was considered lesser (theft and possible complicity, though she was not executed).

In accordance with Danish-Icelandic law at the time, Friðrik Sigurðsson and Agnes Magnúsdóttir were sentenced to death. Their public execution by beheading was carried out on 12 January 1830, at Vatnsdalshólar, a historic hill site in Húnavatnssýsla. The execution was performed by a Danish executioner who had traveled to Iceland for the purpose.