SEATTLE - Coast Guard officials say a distress call from a little girl about a possible boater in trouble in Puget Sound is "a mystery to us" two days after it happened, with no evidence whether it was or wasn't a hoax.

The Coast Guard received the distress call from a 6-year-old girl who identified herself as "Kelsey" at 6:13 p.m. Tuesday and said someone she was with, possibly named "Kelly" - was "in the water" and was "not waking up." (Listen to the full radio call.)

But no trace of the girl or the boat was ever found, despite a search involving hundreds of man-hours, helicopters, aircraft and volunteers.

"It's a mystery to us," said Coast Guard Capt. Steve Metruck at a Thursday afternoon news conference. "We don't want to say it was completely a hoax, but we have no other information - no reports of overdues - to start an additional search at this time."

The Coast Guard released a tape of the full radio call, in which a little girl can be heard talking to a Coast Guard dispatcher and responding haltingly to his questions before vanishing from the air.

"They asked her what school she went to. She said she's schooled at home, basically that's all the information we were able to get out of her other than repeating that she was on the water and that her aunt, Kelly, was asleep," said Coast Guard Lt. Ron Owens.

After receiving the distress call, the Coast Guard immediately deployed a helicopter and a small boat to the waters around Fox Island in south Puget Sound in response.

Additional aircraft were brought in Wednesday morning to assist with the search, but the Coast Guard called off the search after confirming no boaters had been reported missing.

Originally, the Coast Guard said they didn't believe the call was a hoax and are still ready to resume the search if more information becomes available.

"It's very frustrating for the first part," Metruck said. "You want to respond to it because there's a child in distress, someone's in the water - and the water here is very unforgiving. And then as it progressed, you get more and more stressed because the person stopped broadcasting and you couldn't get any more information."

But as the hours passed with no results, he said, "Then you move in to - well, is it possibly a hoax? And you don't want to let that creep into your mind, but when you have no more additional information, you're hoping it's a hoax, because then the person's not at risk. Then you go into another stage, which is irritation at that person, and (especially) using a child to perpetrate that - if it was a hoax."

The call came in on VHF Channel 16, which is the channel typically used for boaters to contact the Coast Guard if they are in trouble. The girl didn't say where she was located, but the signal originated in South Puget Sound area, and search efforts were focused around Fox Island just south of the Tacoma Narrows Bridge.

The Coast Guard recently implemented a new radio direction-finding system known as Rescue 21 that allows locations of radio calls to be triangulated, but the call from the young girl was only picked up on one receiver, so an exact location could not be determined.

The Coast Guard searched the area by boat and by air, but did not find anyone believed to be Kelsey.

Anyone who heard the girl on the radio or can provide additional information is urged to contact the Coast Guard Sector Seattle (206) 217-6001.

The Coast Guard says that if the call was hoax, whoever did it could face six years in prison, a $250,000 fine and possible reimbursement of the search costs. Making a hoax call is federal felony.

Tuesday's search cost more than $38,000.