By Ronnie Cohen

SAN FRANCISCO (Reuters) - The U.S. Coast Guard halted its search off northern California on Tuesday for a crippled sailboat and four people reported to have abandoned the vessel, and investigators also were looking into whether the distress call received two days ago was a hoax.

The search operation, suspended as of 9:30 a.m. local time, had covered an area the size of West Virginia but found no signs of the boat or of two adults and two children who were presumed missing since Sunday, Coast Guard spokesman Thomas McKenzie said.

The Coast Guard began scanning the frigid, choppy coastal waters from Half Moon Bay south to Monterey with search vessels and aircraft after receiving a radio message on Sunday afternoon saying a 29-foot sailboat was disabled and taking on water about 65 miles off Point Pillar.

A little more than an hour after the initial distress call, a man identifying himself as the vessel's operator radioed that the four people aboard were abandoning the craft and planned to use a makeshift life raft to stay afloat. The Coast Guard then lost communications with the caller, officials said.

The Coast Guard has so far been unable to identify those said to be aboard the vessel, and there have been no reports of such a group of missing people by relatives, friends, co-workers or neighbors. But the agency for now was continuing to treat the distress call seriously while also examining the possibility of a hoax, McKenzie said.

"We are investigating," he said. "We're not making that determination at this time."

Another Coast Guard spokesman, Mike Lutz, said: "Hopefully, we can find the family safe on shore, and they can explain the situation to us."

The Coast Guard last year twice pursued time-consuming, high-profile searches sparked by distress calls that turned out to have been faked - one in Texas and another in New Jersey.

The latest operation in California was called off after a 42-hour aerial and seaborne search of nearly 20,000 square miles of ocean failed to turn up a single trace of a disabled or sunken vessel or anyone who might have been aboard. Questions about the possibility of a hoax were not a factor, Lutz said.

Adding to doubts raised about the case, the Coast Guard said it has been unable to find a boat registered under the name of the vessel given in the distress call, the Charmblow, or anything close to that name.

Based on the radio calls, the Coast Guard initially had reported that two adults and two young children, both under the age of 8, were believed lost at sea. The San Francisco Chronicle went so far as to identify the victims as a man and woman, their 4-year-old son and his cousin, who was less than 8 years of age.

On Monday, the Coast Guard released a static-filled audio clip from the radio calls, in which a calm, male voice can be heard saying: "Coast Guard, Coast Guard: We are abandoning ship. This is the Charmblow. We are abandoning ship."

So far no one was come forward claiming to recognize the voice.

Last June, the Coast Guard launched a major search across off Sandy Hook, New Jersey, after receiving a distress call reporting the explosion of a yacht with 21 people said to be abandoning that vessel. Officials later determined the call was a fabrication that likely originated from land rather than sea.

Authorities later said they were investigating links between the New Jersey hoax and another faked distress call in May near Houston, Texas.

The Coast Guard responds to 64 search and rescue cases and saves an average of 12 lives a day, according to its website.

(Reporting by Ronnie Cohen; Writing by Steve Gorman; Editing by Cynthia Johnston and Leslie Adler)