One of the last radio messages she sent was a frantic one to the U.S. Coast Guard cutter Itasca, steaming through the Pacific Ocean, trying to follow the progress of her small plane on its ambitious, grueling flight.

"We must be on you but cannot see you but gas is running low ..."

It was 50 years ago today that Amelia Earhart's silver Lockheed Electra twin-engine airplane disappeared from sight and radio contact in midmorning as Earhart and her navigator Fred Noonan flew from Lae, New Guinea, to Howland Island on the last leg of a projected round-the-world trip.

By noon on July 2, 1937, the U.S. Navy had begun a massive search for the two aviators that would end in futility and begin one of the most romantic mysteries in aviation history. Popular as the 38-year-old Earhart already was, her disappearance catapulted her to legendary status among 20th-century American adventurers.

Her voyage has been retraced (successfully), her image has been printed on a stamp, and the bright red Lockheed Vega she flew solo across the Atlantic in 1932 -- a record for a woman flier -- has been placed in the Smithsonian's Air and Space Museum.

Earhart's disappearance has obscured many of the facts of her life -- which also vary from source to source. Born in Atchison, Kan., in 1898, she graduated from Hyde Park High School in Chicago and attended Columbia University for a while. One source has her getting hooked on flying when her father took her to an air show in California; another says she got interested when she served as a nurse's aide with the Red Cross in Toronto in 1918. She was not the first licensed woman pilot -- in the late '20s there were slightly more than 100 -- but she was a founding member of an organization of women pilots called the Ninety Nines.

What is certain is that in an era when America was fascinated by what airplanes could do and obsessed with records, she set a number of them. The most famous was the solo flight across the Atlantic, from Newfoundland to Ireland. Others, which now look rather quaint, include the first nonstop flight from Mexico City to Newark in 1935 and the first solo flight from Hawaii to Oakland that same year.

By 1932 she was famous, partly because of the work of George Palmer Putnam, of the publishing family, an energetic promoter of Earhart and the man who became her husband. Opinions ranged on how much Putnam pushed her, but she certainly undertook many of her activities with enthusiasm. She spoke often about air travel and how safe and convenient it was, she visited airports, she wrote books, she designed a line of clothes, she modeled for Vanity Fair and Town and Country, and she lent her name to a line of luggage. She also became the visiting vocational counselor for women at Purdue University, which in 1936 bought her the Electra she attempted to fly around the world.

"There were two things she set out to do," says Claudia Oakes, a Smithsonian Air and Space Museum aeronautics curator. "She wanted to show that women make good pilots and she also set out to show flying was safe."

America loved her. She apparently chafed at the title "Lady Lindy" -- she bore a surprising resemblance to Charles Lindbergh -- but it stuck.

Theories about her disappearance proliferate like variations on folk tales, ranging from the absurd to the provocative but rarely dipping to the mundane.

One of the most popular: Earhart and Noonan were on a secret spying mission for President Franklin Roosevelt. Instead of heading for Howland Island, they headed to the Caroline Islands in the Pacific to observe Japanese military air facilities. Frederick Goerner, author of the 20-year-old book "The Search for Amelia Earhart," made several trips in the early 1960s to the South Pacific to interview islanders -- who said they remember an American man and woman, said to be aviators, being guarded at various places. Goerner surmises that Earhart did crash -- at Mili Atoll in the Marshall Islands, where she was captured by the Japanese, taken to Saipan and held prisoner before probably being executed.

Through the years, people have added their shreds of evidence to this theory. Two former Marines told Goerner that while stationed on a Pacific island in 1944 they were instructed to dig up remains of someone who their superiors cryptically hinted was Earhart. And a retired Air Force veteran insisted that he saw the wreckage of her Electra in 1952 in the Marshall Islands.

Last year, a Houston real estate developer announced he had found not only an eyewitness -- an elderly woman on the island of Saipan -- to the execution of Earhart by Japanese soldiers but also a shred of the blindfold she was wearing before she fell dead into an open grave.

Fueling the spy mission rumors is the fact that Earhart and Putnam were acquainted with the Roosevelts. However, scholars of the FDR era have dismissed the theories. "It's an old legend that a number of people have tried to find evidence for," historian and Roosevelt biographer Arthur Schlesinger Jr. said last year when the developer's story surfaced. "I know no evidence connecting Roosevelt and Earhart in espionage."

One of the more amusing: While Earhart was on the spying mission over the Caroline Islands, the Japanese shot her down and kept her a prisoner of Emperor Hirohito in the Imperial Palace in Tokyo until after World War II. She survived and later turned up in Bedford Village, N.Y., as Irene Bolam, whose name "appeared to be a code which spelled out in degrees and minutes of latitude and longitude the precise location of a tropical beach where Amelia Earhart and Fred Noonan crashed after being shot down," according to a 1970 book, "Amelia Earhart Lives," by former Air Force officers Lt. Col. Joe Klaas and Maj. Joseph Gervais.

When the authors wrote Bolam -- who had actually known Earhart -- that they thought she might be the pilot, she wrote back to them, "Unfortunately your quest has not ended, for I am not she."

Later, when the book came out, Bolam sued the authors, and apparently further publication of the book ceased. The book is dedicated "To you, Amelia Earhart, wherever you are."

The Japanese have long denied that Earhart was held prisoner in Japan.

One of the more intriguing: Patricia Morton, a deputy examiner of foreign service applicants, says she has found a telegram in the National Archives. Sent at the end of World War II from a Japanese prisoner camp to Earhart's husband, it reads: "Camp Liberated. All Well. Volumes to Tell. Love to Mother." It is unsigned.

A theory opening tonight: According to Robert McNamara, the Washington playwright who wrote and directed "Amelia ... Last Flight," which starts a run tonight at Source Theatre's Warehouse space, Earhart "went down in the water and got picked up by the Japanese about 10 to 12 days later." McNamara's extensive research leads him to believe the flotation devices aboard the Electra would have allowed the two aviators "to float like a cork in water" for a while. "I think she was in a POW camp for at least a year," McNamara says. He believes Earhart was taken to either Guam or Saipan, where she died of dysentery. Noonan, he believes, was executed.

McNamara also believes Earhart was doing some kind of spying: "We used to ask civilians going into strange parts of the world to keep their eyes open."

The ham radio operator's long-long-distance pickup: This is a variation on a basic theory. There is a man who says that as a boy in Oakland -- the starting point of Earhart's round-the-world flight -- he monitored her by ham radio and picked up a transmission from Earhart in which she said she was broadcasting from the wing of the plane (apparently after it went down) and could see a Japanese ship coming toward her.

What probably happened: "I think in essence she ran out of fuel and the plane went down and she and Noonan either drowned or were killed on impact or died of exposure," said Oakes, the Air and Space Museum curator.

According to Oakes, the route Earhart chose, because it was so close to the Equator, would end up being a 29,000-mile trek that she possibly wasn't as mentally and physically rested for as she should have been. "It was hard flying," said Oakes. "The idea of flying an airplane all the time, paying mental and physical attention to all you were doing all the time -- you weren't putting the plane on autopilot."

Earhart sent dispatches from stops between Oakland, which she left in late May, and New Guinea, which she reached on June 30. "When you read all the things she sent back from various points," Oakes said, "by the time she got to New Guinea, she seemed really tired."

Oakes said that during a 1979 interview with the now-deceased aviator Jacqueline Cochran, a close friend of Earhart's, Cochran said she tried to talk Earhart out of the trip. Oakes quotes Cochran saying of Earhart, "She was a lousy navigator and she was tired -- too tired to make the trip."

Earhart, according to Oakes, had been doing a lot of flying and personal appearances as well as working with her husband on building a house in Wyoming.

Some unfortunate technical problems may have come into play. A trailing antenna, which might have given her better radio contact with the Itasca, was left behind because Earhart didn't know how to use it, according to Oakes.

But more important, there is now evidence that her maps were off. This theory is now being advanced by a local Earhart biographer, Doris Rich, as well.

"Just the smallest miscalculation could put you hundreds of miles off course, and you can't just land anywhere in the Pacific Ocean," Oakes said.

Earhart herself was well aware of the dangers of that last leg of the trip, as she hinted ominously in the last dispatch she sent from New Guinea. In "Last Flight," a collection of her writings put together by her husband after her death, she said: "... The whole width of the world has passed behind us -- except this broad ocean. I shall be glad when we have the hazards of its navigation behind us."