The Big Island search for a missing tour plane and three people aboard came up empty Wednesday, with the only clue coming from a brief and faint signal.

Rescue workers called off the ground search for the single-engine Cessna 172, and the air search involving at least 12 aircraft will continue Thursday, according to Hawaii County Civil Defense.

The radio signal was not strong enough to locate the plane, which carried two sightseers and a pilot. It was last seen Tuesday over Kilauea volcano, said Coast Guard Petty Officer Luke Clayton.

The Japanese consulate in Honolulu said there was a possibility the passengers were Japanese nationals.

The aircraft, operated by Island Hoppers, departed from Kona International Airport at 10:25 a.m. but failed to return at its scheduled time of 1:30 p.m., the Federal Aviation Administration said.

There was no distress signal or radio contact from the plane before it vanished, the FAA said.

The Coast Guard said it has scoured more than 2,500 square miles in the search, an area four times the size of the island of Oahu.

"There are some areas socked in, but there are other areas that we can still search," said Deputy Fire Chief Glen Honda. "We always hope for a good resolution."

The missing Cessna was built in 1974 and issued a flying certificate last October, according to FAA records.

Meanwhile, Island Hoppers has suspended its tours and said it was focused on the safe return of the pilot and two passengers. The company would not identify the tourists or pilot.

Island Hoppers has been operating tours since 1986 and has never had a fatality, according to its Web site.

In April, a pilot aboard one of the company's planes was forced to make an emergency landing on a Big Island highway after his plane experienced mechanical problems. None of the six people on board was injured.

Company officials would not confirm if the plane in April's incident was the one that was missing.

On April 18, 2004, one of the company's single-engine Piper Warrior crashed in South Kona, severely injuring pilot Jelica Matic and a Portsmouth, Ohio couple who later sued the company and pilot.

Dallas and Catherine Ratcliff claimed they were stranded at the crash site for more than five hours because the plane's emergency beacon failed to work and the pilot gave 911 operators an inaccurate location of the crash site.

The couple also alleged that the defendants misrepresented Matic's experience and employment history to discourage them from canceling their flight.

Matic had worked as a pilot in Hawaii for only 11 days before the crash. She had previously worked as a flight instructor in Arizona.

The National Transportation Safety Board ruled in March 2006 that pilot error was the probable cause of the crash.