FRESNO, Calif. -- The airtanker was based out of the Hollister Air Attack Base and had been fighting the Dog Rock Fire near El Portal when officials lost contact with it late Tuesday afternoon.
The pilot's name is not being released until all immediate family has been notified.
"This crash underscores just how inherently dangerous wildland firefighting is and the job is further compounded this year by extreme fire conditions," said Chief Ken Pimlott, CAL FIRE director. "We have secured the crash site and will be cooperating with the NTSB on their investigation."
"Our thoughts and prayers are with the pilot's family during this difficult time," said Jeff Cavarra, program director for DynCorp International.
The plane went down at about 4:30 p.m. within a mile of the park's west entrance, Yosemite spokesman Scott Gediman said. Rescue crews were working their way through difficult terrain to reach the plane.
VIDEO: Bystanders witness horrific airtanker crash
The airplane, manufactured in 2001, is an S-2T air tanker, which is flown by a single pilot and has no other crew members. The tanker uses twin turbine engines and is capable of carrying 1,200 gallons of fire retardant, said another CalFire spokesman, Daniel Berlant.
"All the tourists and residents were being turned away," Lewis said, when Michael reported that he had just witnessed "a bomber collide into the river canyon, the canyon wall, and watched it explode in flames and reported there was plane debris landing in the highway."
The canyon wall is above the highway and the Merced River, Lewis said.
"It's almost vertical canyon walls," Lewis said, "and the road was cut in 100 years ago right along the river. Anything that falls from the top is going to fall right on the roadway."
The fire had broken out about 90 minutes earlier Tuesday near Route 140, which leads into the heart of the park. It had grown to about 130 acres by Tuesday evening and forced the evacuation of several dozen homes near the community of Foresta.
The National Transportation Safety Board and Federal Aviation Administration were investigating the crash and were expected to arrive at the crash site Wednesday morning, FAA spokesman Ian Gregor said.
The last time a CalFire air tanker crashed was in 2001, when two tankers collided while fighting a fire in Mendocino County, killing both pilots, Berlant said.
The agency had another plane crash in 2006, when a fire battalion chief and a pilot were killed while observing a fire in a two-seat plane in Tulare County.
Stay with ABC7 News for more on this developing story.