They were three very different men.

Kevin O'Connor was the motivator -- overseeing a staff of more than a dozen biologists and field workers with enthusiasm and ambition. Clu Cotter was the athletic and adventurous wanderer -- ready to take a detour at any moment just for the fun of it. Tom Stolberg was the eccentric jack-of-all trades -- dabbling in everything from cooking to medieval warfare to sewing.

But they also had one thing in common: a love for the outdoors and a fascination with wildlife.

They worked in the Fresno office of the California Department of Fish and Game -- which is responsible for 12 counties in Central California -- and were three of the office's four wildlife specialists. They were tracking deer herds on Jan. 5 when their helicopter clipped a power line and crashed into the base of a large tree, killing them. The pilot, 70-year-old Dennis Donovan of Palm Springs, also died.

The National Transportation Safety Board is expected to issue a preliminary report on the accident this week.

Family members and colleagues in the tight-knit Fish and Game office -- which is often described as a family itself -- are grieving their loss. But they take some comfort in knowing that the men died doing what they loved best: embracing adventure.

Here are their stories.

Cotter: An 'adventurous spirit'

When Ed Reagan, an avid cyclist, wanted to take an all-night trip into the foothills near Tollhouse, he knew who to call: his friend Clu Cotter. It was going to be a full moon, he told Cotter; so why not bike up there at dusk and come back down at night? Cotter didn't hesitate. He was in.

Cotter, 48, was trim, fit and curly haired. He was a devoted family man who always had a calm demeanor, his friends say.

But he was no desk jockey. With "legs like a locomotive," as one friend put it, Cotter was well-known in the Central Valley bicycling community as one of the area's most competitive mountain and distance bikers.

A 90-mile day trip from the Valley floor to Grant Grove in Kings Canyon National Park? Cotter was up for it. A 10-day romp around Belgium tracing the country's famous biking routes? That was Cotter's idea. A trip to Oregon to compete in a muddy and grueling cyclocross race -- Cotter was there.

"When you looked down at your cell phone and saw that it was Clu, you smiled because you knew an adventure was coming," Reagan said.

During a hike up Half Dome in Yosemite National Park in July 1985, Cotter used his outdoor survival skills to help rescue a group of hikers caught in a lighting storm, according to Cotter's longtime friend Jim Ryan and a Bee story about the incident. Two of the hikers in the group were killed by a lightning strike and three others were injured. Cotter and his friends helped keep the survivors warm until helicopters rescued them around midnight.

Cotter grew up in Marin County and graduated in 1990 from California Polytechnic State University, San Luis Obispo, with a degree in ecology and systematic biology. He moved to the Valley to work as a wildlife technician for the U.S. Forest Service. He later conducted endangered species surveys and led habitat restoration projects for the state Department of Water Resources. In 1999, he joined Fish and Game as an associate wildlife biologist.

Cotter spent much of his time working in the eastern Fresno County foothills and mountains studying deer and rare carnivores such as the Sierra red fox, said Jeff Single, manager of Fish and Game's Fresno office. Any chance to hike, ski or bike to some distant spot was welcome news to Cotter.