RENO, Nevada, Sept 12 (Reuters) - The Nevada Air National Guard dispatched a Blackhawk helicopter to a canyon on Wednesday afternoon after receiving a "credible" tip that missing millionaire adventurer Steven Fossett's plane may have crashed there.

Yet the search, as well as checks of a separate possible site near the California border and of a third possible wreck location have not yielded immediate results, said Chuck Allen, a spokesman for the Nevada Department of Public Safety.

Fossett, the first person to fly a balloon solo around the world in 2002, disappeared a week ago on Monday after taking off alone in a small private plane in Nevada.

Officials say the tip came from a former Civil Air Patrol member whose friend told him he saw an aircraft with markings similar to Fossett's make a diving turn into the canyon and not exiting. A second witness also saw the descending aircraft on the day Fossett went missing.

"Ground crews are now looking at the Nevada site," Allen said of the location 20 miles (32 km) west of Minden.

Officials say the area was searched before but thick pine nut trees and brush could have obscured any crash from view. Searchers are hoping eyes on the ground will see what aviators cannot.

Allen said it was not clear why the tip would have come in more than a week later amid wide international publicity. Fossett is also known for making a record-setting solo nonstop airplane flight around the world in 2005.

The spokesman said officials determined that another possible wreck site was not that of Fossett's plane, which would be the seventh wreck from previous accidents found during the extensive source.

Officials have led the search across a largely barren area of Nevada with planes and helicopters in the sky, foot patrols on the ground, divers in a lake and analysts poring over maps and data. (Reporting by Willie Albright)

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