OXFORD, Penn. - There are many people with theories about what happened to Amelia Earhart. But few stir up more excitement - or more ire - than Ric Gillespie.

The longstanding official theory is that the famed pilot and her navigator, Fred Noonan, ran out of gas and crashed into deep ocean waters northwest of Howland Island, a tiny speck in the South Pacific that the pair missed while attempting a round-the-world flight in 1937.

Since 1989, Gillespie and The International Group for Historic Aircraft Recovery, or TIGHAR, have been testing another theory - and they've headed back to the region this month. They surmise that Earhart made an emergency landing on a flat stretch of coral reef off what was then known as Gardner Island, southwest of Howland. And they've raised millions in private funds to finance several treks to the distant atoll, now called Nikumaroro.

Set to arrive this weekend, the TIGHAR team now wants to check an anomaly seen in sonar imaging on an underwater cliff where the reef drops off.

Could it be the fuselage of Earhart's Lockheed Electra 10E airplane?

Gillespie makes no promises: "There's no guarantee of success."

He's far from the only one looking for Earhart.

An Australian researcher thinks wreckage spotted by members of his country's military years ago on a Papua New Guinea island could be hers. Others are investigating local island lore that Earhart and Noonan crash landed on Mili Atoll, 800 miles northwest of Howland, and died in Japanese hands.

Various teams who believe the crashed-and-sank theory - an explanation supported by curators at the Smithsonian's Air and Space Museum - have tried to find the plane using sophisticated equipment to scan the ocean floor. No one has found a verified plane part or bone fragment.

But Gillespie said he and his team are building their case, slowly but surely.

He has his admirers. In 2012, then Secretary of State Hillary Clinton recognized Gillespie at a reception honoring Earhart. In a letter to him, she said, "This great adventure embodies the very hope, ingenuity and boundless optimism of the American spirit" - a reference to the expedition that year in which TIGHAR collected several underwater sonar images.

But there've been disappointments and controversy, too.

There was the filing cabinet discovered on Nikumaroro that the team thought came from Earhart's plane but later linked to a military aircraft. The team also excavated a grave that turned up bones, not of the famous pilot but of a tiny infant.

One of TIGHAR's more controversial finds is a piece of metal, likely from an airplane, which the team found at Nikumaroro in the early 1990s. Gillespie's latest theory is that it's a patch that covered a window on Earhart's plane. Many critics dispute that, though Gillespie has at least one notable supporter - MIT engineering professor Thomas Eagar who thinks it may be "the real thing."

In England, the team also found records of human bones found long ago on the island. The bones are missing, but Gillespie says modern-day analysis of measurements indicate they could've come from a woman of European descent.

None of it is definitive proof, he and his team realize.

"We have compiled a preponderance of evidence suggesting - not proving - that our hypothesis is true," says Tom King, TIGHAR's lead archaeologist.

Some critics insist that Gillespie has found nothing remotely tied to Earhart - and that remnants on the island are more likely from a former Coast Guard station or from islanders who settled on Nikumaroro after Earhart's disappearance until the mid-1960s.

Then there's Tim Mellon, a one-time supporter and now critic, who thinks quite the opposite - that Gillespie knows more than he reveals. Two years ago, Mellon accused Gillespie in an unsuccessful lawsuit of hiding the fact that he'd found Earhart's plane so Mellon would donate more than $1 million in stock to help fund the 2012 expedition. A judge rejected Mellon's appeal last month, but he's sticking to his assessment of Gillespie.

"It's a business for him . even though he calls it a charity," Mellon said in a telephone interview.

Now Mellon has filed a complaint with the IRS, claiming that TIGHAR is violating nonprofit guidelines. Already, public records show Gillespie has a state tax delinquency in Delaware for more than $55,000 - an amount Gillespie's wife and TIGHAR co-founder, Pat Thrasher, says they're working to pay back after getting into debt to pay for a defense in the Mellon lawsuit.