Gosia Wozniacka / AP

A hiker takes a photo from the top of Vernal Fall in Yosemite in July 2011.

A 10-year-old boy is dead and a 6-year-old is missing after they were swept away in the Merced River in Yosemite National Park on Wednesday, a park spokesman said. 

The boys were part of a family group from Southern California that hiked about a mile on the Mist Trail to Vernal Fall before stopping at a footbridge around 3 p.m. to hop in the Merced River. 

But the boys were swept away in the strong current. The 10-year-old was pulled out 150 yards downstream and another park visitor administered CPR.

The boy was pronounced dead after further resuscitation efforts by park rangers were unsuccessful. 

A search for the 6-year-old was under way between the footbridge and the trailhead, a distance of about eight-tenths of a mile, according to park spokesman Scott Gediman.

Gediman told NBC News about 20 national park rangers were looking Thursday afternoon.

The part of the river the boys were in has lots of boulders, some the size of a house, Yosemite spokesperson Kari Cobb told NBC News. 

"That portion of the river is a very rough portion," she said. "It's not uncommon for people to go down to that part of the river." 

She told Reuters that the Merced was running "extremely low" but that the current still was swift.

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Gediman said he was not sure if the boys were brothers but that they were with a group of extended family and friends. 

The boys' names have not been released. 

The Mist Trail is one of the most popular hiking trails in Yosemite National Park, which is in the Sierra Nevada east of San Francisco and encompasses 1,200 square miles. 

In July 2011, three visitors to the park were swept over the falls to their deaths. One body was recovered several weeks after the accident, and the others were found in November and December. 

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