(CNN) -- Half way to a popular waterfall at Yosemite National Park, the 15 members of a Southern California family decided to take a break from the trail at a footbridge that crosses the boulder-strewn Merced River.
Somehow, two boys -- whether dipping their feet in the water or wading -- were swept away Wednesday afternoon by the deceptive current, park officials said.
The 10-year-old drowned and a 6-year-old was missing, prompting a search and rescue effort.
The boys are either half-brothers or stepbrothers, officials said.
The tragedy occurred during the park's busiest month, when 750,000 visitors enjoy the verdant national treasure that is the size of Rhode Island.
It was not clear whether the family, hiking on Mist Trail from the Happy Isles Trailhead, intended to go all the way to spectacular Vernal Fall, less than a mile from the footbridge.
The unidentified 10-year-old became the park's third drowning victim of 2012, said ranger and park spokesman Scott Gediman. Efforts to resuscitate the youth were unsuccessful.
A 57-year-old man drowned this year in the Merced River in the western portion of Yosemite and another man died on the south fork of the river in the Wawona area.
Although there was no sign at the footbridge, visitors pass a water warning sign at the trailhead, Gediman said Thursday.
"Yosemite is a wild place by definition," the ranger told CNN, adding parents should assess each situation. "We try to provide the best and most accurate information we can to people."
Although river levels are lower this year because of a low snow pack, the Merced flows rapidly through a V-shaped canyon and chute channels, Gediman said. While the river is only several inches deep at the bank, the middle of the river can be four to five feet deep in places and thick with boulders.
"Whether it is a high water level or low, the Merced River flows year round and the current can pull anybody in," said Gediman.
Nine people have died at Yosemite thus far this year, officials said. One visitor was killed when a tree fell and another died in a rock-climbing accident.
Of the 20 deaths in 2011, seven were water-related, according to Gediman.
In July 2011, a young man lost his footing, slipping close to the edge of the Vernal Fall waterfall. A female companion frantically grabbed for him but stumbled. Another hiker followed and the three died after they were swept over the powerful 317-foot falls.
One body was found in August and two others were recovered in November and December 2011.
Yosemite offers visitors spectacular views of mountains, falls and rivers, but park officials warn visitors to be careful.
Rangers say some visitors partake in dangerous practices such as hiking treacherous trails in flip-flops, climbing over safety rails to take better pictures or swimming perilously close to waterfalls.
Water is a major attraction at Yosemite, and most visitors swim and otherwise safely enjoy its lakes and rivers, Gediman said.
Witnesses told park authorities that the three hikers who were swept over the waterfall in July 2011 had climbed over a safety rail.
"We don't station a ranger in every possible dangerous place that's out there," park ranger Kari Cobb said last year. "People have to come here and realize that Yosemite is nature, and it is a very wild place."
CNN's Michael Martinez and Casey Wian contributed to this report.