HILO, Hawai'I — A ground search team sent to hunt for the source of a brief radio signal in the Hakalau area has been called back after finding nothing, but the aerial search continues for a missing tour plane with a pilot and two Japanese nationals on board.

The pilot was identified as Katsuhiro Takahashi, an experienced pilot with the Island Hoppers tour company.

A U.S. Coast Guard C-130 search plane flying above the Pi'ihonua area early this morning detected a brief radio signal believed to be from the missing Island Hoppers tour flight, but the transmission was too brief to allow crews to find the plane, fire officials said.

Deputy Fire Chief Glen Honda said a ground search team also failed to locate the source, and it was unclear if the brief transmission came from the Cessna 172 missing since Tuesday afternoon.

Honda said a fisherman reported debris this morning about a mile off the coast northwest of South Point, but a Coast Guard aerial search of a 10-mile-square ocean area turned up nothing.

Earlier, Honda had said the radio transmission might be helpful, but was imprecise. "This signal where they picked it up could have been bouncing off of something else, and that's why they picked it up in that area. It doesn't necessarily designate that that's where the locating device is."

The emergency locator transmitters in some cases have been active for three or four days, but that depends on the type of transmitter the Cessna has, said U.S. Coast Guard Lt. John Titchen.

"Generally how it works is the signal is constant, and so we are considering other options here in terms of trying to listen for it, but we're hopeful that we will find it as we continue flying the rest of today," he said.

An overnight search turned up no visible trace of the tour flight that disappeared yesterday afternoon after passing over Kilauea volcano, the U.S. Coast Guard said this morning.

The search this morning encompassed much of the Big Island, with two fire department helicopters resuming the search at daybreak of the areas around Na'alehu, Hawaiian Ocean View Estates and Honomalino in South Kona.

Up to a dozen aircraft joined ground crews in the search today, said Titchen.

The search grid is centered roughly around the last known position of the plane near Kilauea, but searchers have also studied the routes Island Hoppers pilots customarily follow on company tours to focus on that path as well, he said.

The company routinely flies its tours clockwise around the perimeter of the Big Island, leaving Kona, passing over North Kohala, traveling down the Hamakua Coast through Hilo before circling the Hawai'i Volcanoes National Park area. From there the flights generally pass over the Ka'u and South Point area before traveling north back to Kona.

"We've got a pretty good idea of where they could be, and that's what we're looking at now," Titchen said. "We searched that route, (the fire department) has searched that route, and in addition we've scoured the area around the last known position."

The search also traced the route from the volcano around South Point and up to Kona, which has sometimes been a dangerous area for aircraft, Titchen said.

At a news conference this afternoon in Honolulu, Titchen said much of the search focused on the Ka'u Forest Reserve. Earlier today, visibility there had been poor but it was better in the afternoon, he said.

"We're confident we have the area saturated," Titchen said.

He said there were a total of 12 aircraft involved in the search: a Coast Guard C-130, two Hawaii County helicopters, two Civil Air Patrol aircraft, and volunteer aircraft from tour compoanies and flight schools.

Another Island Hoppers pilot and two passengers were seriously injured on April 18, 2004, when their Piper Warrior tour plane struck a hillside near Miloli'i and caught fire.

The National Transportation Safety Board found that pilot Jelica Matic had flown around the Big Island fewer than a dozen times, and was relatively inexperienced with the aircraft she was using.

Matic told investigators the Island Hoppers plane was forced down by a powerful downdraft. She and her two passengers suffered burns from the fire.

Earlier this year, on April 16, another Island Hoppers flight carrying six tourists made an emergency landing on a highway about a mile from Kalapana, but no one was injured in that incident.

The weather for the search has been excellent with unrestricted visibility, and "we're hopeful, and the search will continue for as long as we have a reasonable search area and good conditions," Titchen said.

The search includes the Coast Guard C-130, a Coast Guard Dolphin HH-60 helicopter, and the two Big Island Fire Department Helicopters and six other aircaft from various tour companies including Blue Hawaii and Mauna Loa Helicopters.

This morning, a Civil Air Patrol airplane was searching in the Kalapana area, and a Coast Guard C-130 was conducting a high-altitude search over the southern and southwest end of the island, Honda said. The C-130 conducted the overnight search.

Honda said the brief signal was detected at about 12:30 a.m., and was apparently just a single tone.

Coast Guard Petty officer Luke Clayton said an HH-65 Dolphin helicopter from the Coast Guard Air Station at Barber's Point on O'ahu would join in the search.

"We have been searching continuously," since receiving word that the plane went down, Clayton said.

The single-engine Cessna 172 operated by Island Hoppers departed from Kona airport on a tour flight headed clockwise around the Big Island, and was last seen by another pilot with the same tour company, said Ian Gregor, spokesman for the Federal Aviation Administration.

The plane left Kona at about 10:15 a.m. yesterday after filing a flight plan using visual flight rules, and was last seen at 12:45 p.m., Gregor said. The flight was due back in Kona at about 1:30 p.m., he said.

"When it didn't return to Kona on time, the company reported it missing," prompting the search, Gregor said.

Wendy Hart, general manager of Island Hoppers, issued the following news release at noon today regarding the missing plane:

"An Island Hoppers tour plane carrying two passengers and one pilot did not return at the scheduled time yesterday. All appropriate officials and agencies were notified. Full search efforts have been under way since early yesterday.

"As all of our efforts and thoughts are focused on the safe return of our passengers and pilot, we appreciate the press media's concern and patience. We will issue further statements as information becomes available.

"Mahalo and thank you for your patience and concern."

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

"We are exhausting the search area with as many resources as we can bring to bear," said Lt. Cmdr. Kevin Floyd, supervisor of the Fourteenth Coast Guard District Command Center in Honolulu. "We have great weather along the coast and slightly inland, but our air crews are reporting more difficult conditions near Mauna Kea, where we are concentrating our search efforts."

The Coast Guard will answer questions with media at 3:30 p.m. today at the U.S. Coast Guard Pier 4 building in downtown Honolulu.