On October 20, 1846, an exceptionally early and heavy snowstorm dumped foot after foot of cement-like snow on the Sierra Nevada mountains, trapping 81 members of the ill-fated Donner Party, more than half of them children. Their five-month ordeal is one of the most infamous in American history, haunted by the fact that roughly half of the 45 survivors resorted to cannibalism when all other food sources—including boiled bark and leather—were depleted.

But less well-known is the story of how rescuers marched bravely into the “camps of death” multiple times to lead (and in some cases, carry) the starving Donner Party survivors to safety. Among those rescuers was James Reed, who had been expelled from the Donner Party for killing a man earlier on the trek, but without whom they may have all perished on the frozen mountain.

The snow kept falling for weeks, burying the makeshift cabins of the Donner Party. The frigid weather was unrelenting and the emigrants had exhausted nearly everything edible—oxen and their hides, pet dogs, field mice, even leather shoelaces. By mid-December, when it became clear that the weather wasn’t going to break, 15 of the strongest and healthiest men and women strapped on rudimentary snowshoes and set out to cross the summit and find help.

The escape party became known as the “Forlorn Hope.”

“They soon found out that it was going to be an incredibly difficult task,” says Michael Wallis, author of The Best Land Under Heaven: The Donner Party in the Age of Manifest Destiny. “It was horrible climbing that great divide with frostbitten hands and feet, weakened by hunger, panting for air. The sun reflected off the snow and ice and burned the corneas of their eyes. The pain became unbearable.”

One by one, members of the Forlorn Hope died from exposure, and their starving comrades became the first of the Donner Party to break the taboo of eating the dead. The life-sustaining flesh gave them the strength to push on, but that act also inspired a far worse crime. A man named William Foster shot and killed two Miwok tribesmen who were accompanying the Forlorn Hope as guides, and the survivors ate the men as if they were any other “animal.”

Ironically, it was Miwok villagers who made first contact with the seven surviving members of the Forlorn Hope and fed and clothed them when the half-dead escape party stumbled into the valley below. The harrowing journey from Donner Lake had taken 33 days.

Word spread quickly of the starving families trapped at Truckee Lake (now Donner Lake) in the Sierra. The first rescue party, known as the First Relief, departed from Sutter’s Fort near modern-day Sacramento. Sutter’s Fort was a mini-empire run by Swiss immigrant John Sutter, who enslaved local Miwok and Nisenan peoples to work his land.

There were only seven men with the first relief rescue party, and the going was slow and dangerous.

“In some places, the snow around them was 30 feet deep,” says Wallis, “and they had to ditch supplies along the way because their packs were too heavy. They needed relief as much as the people they were rescuing.”

When the First Relief finally arrived at the Donner Party camps, they had little food left to distribute to the desperate families, but they offered to lead the strongest back down the mountain to safety. Sadly, many of those individuals didn’t survive the journey.

Meanwhile, James Reed was frantically trying to raise money for a second expedition to save his wife and children. Reed, a lawyer from Illinois, was one of the organizers of the Donner Party expedition. In a heated exchange with a teamster (someone who drives oxen) named John Snyder, Reed stabbed Snyder in the chest and killed him along the trail. While some wanted to hang Reed on the spot, the pioneers chose to banish him from the party.

Traveling alone, Reed crossed the Sierra before the snows fell and was in California when he received the terrible news of his family’s fate. Reed was temporarily sidetracked from the rescue effort by the outbreak of armed clashes with Mexico, which still governed Alta California. In January of 1847, Reed fought in the Battle of Santa Clara, and was able to recruit some fellow soldiers and gather supplies for the second relief mission to the mountains.

By this point, cannibalism was widespread at the Donner Party camps as no other sustenance was available. Reed’s rescue party was able to evacuate 17 people, including Reed’s own family and most of the Donner family.

“I can’t overemphasize the importance of James Reed,” says Wallis. “If it wasn’t for Reed, the rest of the Donner Party would have died.”

The ordeal wasn’t over, though. Over the course of more than two months, a total of four missions sent rescuers trekking into the Sierra to evacuate everyone they could, including the sickly and the young.

John Stark was part of the Third Relief and was dubbed a hero for his actions in March 1847, when he and two other rescuers saved 11 people, including nine children, who had been left behind. Stark, a hardy settler, carried the children two at a time down the mountain. It was painfully slow and difficult, but all nine of the kids survived. One of them later credited the miraculous rescue to “nobody but God and Stark and the Virgin Mary.”

During the fourth and final relief mission in mid-April of 1847, the rescuers only found one survivor among the gruesome remains of half-consumed corpses and severed limbs. The last man alive was Lewis Keseberg, an irascible German immigrant who was found in possession of Donner family gold and heirlooms. Keseberg was put on trial for killing and eating six fellow survivors, including Tamsen Donner, wife of George Donner, one of the organizers of the doomed expedition. Keseberg was ultimately acquitted, but was forever cast as a blood-thirsty cannibal.

Only 45 of the original 81 members of the Donner Party survived, 32 of them children. Most were physically scarred from frostbite and malnutrition, and psychologically disturbed by the horrors of what they experienced in the camps, and what they had to do to survive. The Reed family, however, went on to prosper as one of the original settlers of San Jose, California.