Investigators are trying to determine how a man plunged more than 4,000 feet to his death while climbing the Half Dome summit in Yosemite National Park.

It was the 17th fatality in the park this year, making 2011 one of the deadliest years in at least a decade.

Rock climbers at the bottom of Half Dome heard the man scream and watched him fall off the face of the picturesque peak at about 6:30 p.m. Monday, Yosemite spokesman Scott Gediman said Tuesday.

Because of the oncoming darkness, rescue workers were not able to search for him until Tuesday morning, Gediman said. A helicopter crew spotted the man's body in the morning and rangers reached him at 1 p.m. Officials have been unable to identify the man.

Mariposa County medical examiners are expected to perform an autopsy today.

The man appeared to have been properly equipped, and conditions Monday were warm and dry. He is believed to have used Half Dome's cables to scale the summit.

"At this point, it looks like it was accidental," Gediman said, saying that investigators do not believe foul play was involved.

Meanwhile, park rangers have been interviewing other hikers and examining a few possessions believed to have belonged to the victim for hints to his identity. Rangers have not fielded any missing persons reports matching the man's description, but that is not unusual in these types of situations, Gediman said.

A visitor from, say, Colorado, might tell friends and family that they're hiking for a week in Yosemite, Gediman said. So it might be a few days before the man is reported missing - or identified.

Yosemite has averaged about a dozen deaths annually over the past decade, according to park officials.

It is the second death within a month on Half Dome. On July 31, Hayley LaFlamme, 26, of San Ramon plummeted 600 feet to her death while climbing Half Dome after stormy, wet weather.

LaFlamme's death was the first since the park instituted a permit system last summer that limits the number of Half Dome climbers to 400 each day. Previously, up to 1,200 people a day were squeezing onto the cables.

There also were fatal falls at Half Dome in 2006, 2007 and 2009.

Many of this year's accidents stem from the record Sierra Nevada snowpack, which has inflated scenic summer rivers into raging whitewater.

In July, three Central Valley residents were swept over Vernal Fall as dozens of visitors watched in horror. The three ignored warning signs and scaled the safety railing atop the popular Mist Trail. They were standing in knee-high water when, one by one, they slipped and plunged 317 feet over the waterfall's edge.

In May, a Texas man slipped on the granite steps below Vernal Fall and slid into the Merced River, where he drowned. And in June, two Southern California men were swept from the Wapama Falls bridge near Hetch Hetchy Reservoir.

But Gediman said there has been no pattern to this year's higher-than-average number of fatalities. "There has been no definitive explanation for them," he said.