Peter Lassen was born in Farum, Denmark, and immigrated in 1830 to Boston, Massachusetts. Born to Johanne Sophie Westergaard and Lars Nielsen, Peter was baptized at the age of one week on December 7, 1800. The patronymic surname Larsen, meaning son of Lars, was applied in his naming and was common among Danish farmers at that time. Lassen used several different spellings while living in Denmark and in Copenhagen, sometimes preferring to be called Peter Larsen Farum, after his birthplace. When he was seventeen, Lassen moved to Kalundborg to learn blacksmithing. He apprenticed under his uncle, Christian Nielsen until 1823, when he moved to Copenhagen to work under a master blacksmith named Sigersted.
Pioneering California
In 1840, he immigrated to California and became a rancher. Peter Lassen traveled from Keyetsville, Missouri, to Westport, Missouri, a "common place of rendezvous for overland parties". The expedition Lassen joined was made up of missionaries and adventurers, some on their way to California; others headed to the Columbia River basin in Oregon. Lassen met the ship Lausanne on the Willamette River in Oregon. He sailed on July 3, 1840, boarding for passage to Bodega Bay, in Sonoma County, California. The travelers were invited to touch land in California at Fort Ross, and stayed on there until they could procure some horses to travel to Sutter's Fort. Lassen arrived at Sutter's Fort in August 1840, with William Wiggins, who had traveled with him from Westport, over land and by sea for nearly fifteen months starting in May 1839. John Sutter happily welcomed the men and traveled with them from his colony, known at the time as New Helvetia, to Yerba Buena and Alviso, California, in a small sailboat. While it is likely he was working as a blacksmith during his early years in California, Lassen traveled to Santa Cruz in 1841, where he oversaw the construction of a sawmill owned by Isaac Graham on Zayante Creek. Lassen was paid 100 mules for his efforts. From the winter of 1840- 1841 Lassen returned to San Jose, with another pioneer blacksmith, William "Guillermo" Gulnac; the two travelling to the upper San Joaquin Valley in the Spring of 1841. In July 1842, the men arrived at "Rancho Campo de los Franceses" but just missed connecting with French-Canadian trappers who worked for the Hudson's Bay Company. Lassen pushed on with his livestock, and instead spent the next few years camped in a hut along the Cosumnes River, working as a travelling blacksmith.
Lassen established the Lassen Cutoff of the California Trail, which left the main trail near the modern-day Rye Patch Reservoir and crossed a desolate section of what is now northwestern Nevada, including the Black Rock Desert. His cutoff continued to Goose Lake in northeastern California, and then followed the Pit River into California's Central Valley. Portions of this trail were particularly arduous, and many of its early travelers greatly regretted choosing it. The route was extensively traveled during the years 1848 to 1853, but because of the hardships of the route, the trail was little used thereafter. The Applegate Trail also traveled from Rye Patch Reservoir to Goose Lake. John D. Unruh, Jr., writes of Lassen's first attempt at using his cutoff, rescued from disaster by a group of well-supplied emigrants from Oregon who helped him reach his ranch:
Here the wily Dane orchestrated a meeting wherein the emigrants supposedly endorsed him as a guide and warmly praised his cutoffs. This deceptive endorsement was rushed eastward to be printed in newspapers and influence credulous forty-niners hell-bent for the gold fields. Planning carefully, Lassen also dispatched agents to divert forty-niners onto the cutoff and to set up trail advertisements with the reassuring message that the diggings were a mere 110 miles ahead... emigration trustingly followed their lead, many foolishly discarding surplus provisions on the assumption that only 110 miles remained. The suffering of those choosing the Lassen Cutoff was severe, for it proved to be some 200 miles longer than either the Carson or Truckee routes.
Lassen was murdered on April 26, 1859 in Clapper Canyon in the Black Rock Mountains as he was traveling in the area, to prospect for silver. He was traveling along with Edward Clapper and Americus Wyatt; Clapper was killed in the same incident, but Wyatt escaped. The circumstances surrounding his death remain mysterious. According to Wyatt, Lassen and Clapper were shot by an unseen sniper while breaking camp. At the time the culprits were widely considered to be Northern Paiute, who were then in a state of unrest, which would soon lead to the Paiute War. However, Wyatt himself, Pit River Indians, and disgruntled emigrants who followed the Lassen trail, have also been suspected. In particular, an investigation at the time disclosed that none of the supplies of Lassen, Clapper or Wyatt had been taken; in the perception of the investigator, leaving the supplies was not normal conduct for a Native American raiding party at that time, and, as a result, Wyatt himself has been suspected as the murderer of Lassen and Clapper. Peter Lassen's grave is in Susanville in Lassen County, California, along the eastern foothills of the Sierra Nevada.
