– On a windy, much warmer day on this lake, Roger Van Surksum snagged the first bison bone with a fishing hook. The fishing guide knew it was no walleye and reeled it in slowly, carefully. The bone was 10 inches long, he said, “as black as the ace of spades.”

He put it in the back of his truck but couldn’t get it out of his mind.

“I had to figure out what it was,” said Van Surksum, 69, standing near the shore of Lake Victoria in Alexandria this week.

He enlisted the help of two divers and, over the summer of 2011, they brought up more than 250 bones from the bottom of the lake on the east edge of Alexandria, in central Minnesota. Then Van Surksum pestered experts for answers. A state archaeologist found that the number and condition of the bison bones hint at an American Indian gathering place — a bison kill site, hundreds or perhaps thousands of years old.

“This could be a really important place … where a part of Minnesota history has been preserved,” said Brian Hoffman, chairman of the anthropology department at Hamline University in St. Paul.

Until recently, little was done to determine the bones’ age or origin. But this semester, a St. Cloud State University class is studying core samples taken from the lake’s bottom, which could reveal clues to the area’s past or even evidence of human settlement.

Van Surksum brought the students out on the ice in late January, to the point marked on his GPS, watching as they collected sediment samples that today bear his name. He remains anxious for answers.

“Who lived here?” said Van Surksum, who began guiding fishing trips after retiring from auto sales. “What kind of human inhabited this area?

“I’m not an archaeologist, but I’ll tell you what: I turned into one.”

In search of walleye, Van Surksum has caught some strange things: blankets, hats, minnow boxes. “A basket full of sunfish,” he said with a half-smile. “They were still wiggling.”

But never before a bone. At first, Van Surksum couldn’t tell whether it had come from a creature — or a person. He brought it to a butcher shop, then an undertaker. Not cow, not human.

“This is not an everyday finding, here,” Van Surksum remembered thinking.

He gave the bone to his daughter-in-law, who was working at St. Cloud State University. She called one day with a match: It was a bison bone.

Turns out, it was far from the only one. Using the GPS coordinates, divers began collecting the bones — 10 on the first trip, more than 100 on the next. Ribs, jaw bones, vertebrae. A photo from 2011 shows how Van Surksum laid them out on his driveway, awaiting a visit from David Mather, the National Register archaeologist for the Minnesota Historical Society.

The find sounded interesting, especially due to its “very unusual setting” under water, Mather said recently. “I was very curious to know whether this was a cultural site or a natural accumulation of bison bones.”

Mather took note of one bone that bore a gouge mark, possibly caused by a human tool. Van Surksum kept trying to steer the zooarchaeologist to the “perfect bones with no cracks in them,” Van Surksum said. But Mather focused on the broken bones. The breaks curved along the bones, known as spiral fractures, showing that the bones were broken when fresh and perhaps pointing to human use.

American Indians — who used bison for food, shelter and tools — broke apart their bones to get the marrow, said Hoffman, the Hamline professor. “So that’s really intriguing. To me, that suggests that this is an archaeological site, an American Indian site.”

After his visit to Alexandria, Mather told Hoffman about the bones, which Van Surksum was storing in apple boxes, under newspaper. The once-black bones, which had begun to fossilize, had started growing lighter. Hoffman picked them up and brought them to St. Paul, with plans to date them.

But after doing an inventory, “basically that’s been the end of it,” Hoffman said. He needs a student to take the project, he said.

In a University of Minnesota lab last week, a dozen St. Cloud State students watched as a curator slowly split open a core from Lake Victoria, revealing a mossy brown layer atop dark, sandy sediment. Tucked into it: a small shell.

“It’ll be interesting finding out from biologists what that is,” said Kate Pound, the geology professor leading the class. The students nodded.

They will take a closer look at the core samples’ grain size and composition, keeping an eye out for anything that might have been brought by people, such as charcoal, Pound said.

“When I read about the bison bones, I thought ‘Ah-ha!’ ” she said. “Now we can use this work to really try and answer a question.”

An excavation of the site could answer tricky questions: When did the bison die? Were they forced over the nearby cliff? Did hunters trap them out on the ice? Or did lower lake levels leave this land exposed?

But despite researchers’ urging, no local museum has shown interest in applying for a grant for the work, Mather said. Investigating the site is more difficult because it’s under water, he said.

While other archaeological sites have been plowed over, the lake has protected this one from development, “a fortuitous thing,” Hoffman said.

At the same time, the site is vulnerable, he added. He’s grateful that divers brought the site to researchers’ attention, but “every time they pull up a bone … we lose a little more of that picture.”

The lake water was murky when divers first went down, Van Surksum said. But zebra mussels have since made it clearer. When the St. Cloud State team took core samples, Van Surksum and his friend lowered a camera, capturing objects that Van Surksum is sure are “so many more bones.”

One afternoon last week, he walked to the lake’s edge with his dog Kuma and pointed to where he was fishing more than four years ago.

“Early in the morning, I’ve stood out here, when it’s really quiet,” he said. “Sometimes you can almost hear the hoofs running over the cliff.

“It’s kind of an echo in the air.”