Three nations shelled out around $160 million and years' worth of work on the underwater search for missing Malaysia Airlines Flight 370. The result: No plane. The only tangible — and arguably most important — clues into what happened to the aircraft have come courtesy of ordinary citizens, who bore the costs themselves.

The deep-sea sonar search for the vanished Boeing 777 was suspended on Tuesday after officials conceded defeat following most expensive, complex aviation search in history.

But while search crews spent years trawling in futility through a remote patch of the Indian Ocean, people wandering along beaches thousands of kilometers (miles) away began spotting pieces of the plane that had washed ashore. Those pieces have provided crucial information to investigators and prompted some to question whether Malaysia, Australia and China — who funded the hunt for the underwater wreckage — missed key opportunities by failing to organize coastal searches for the remnants that drifted to distant shorelines.

"It would have been good to have been getting people looking for debris," said David Griffin, an Australian government oceanographer who worked on an analysis of how the debris drifted in a bid to pinpoint where the plane crashed. "I think that was a job that fell between the cracks of whose responsibility it was."

Since the plane vanished on a flight from Kuala Lumpur to Beijing on March 8, 2014, more than 20 pieces of debris confirmed or believed likely to have come from the aircraft have turned up on beaches along the east coast of Africa and on island nations including Madagascar. All of the parts have been found by local residents and tourists who stumbled upon them, and by an American amateur sleuth who launched his own hunt for debris after working with oceanographers to estimate where bits of the plane may have ended up.

Several family members of Flight 370's passengers asked officials to launch a search along the coastlines for parts of the plane. When their pleas went unheeded, they banded together and traveled to Madagascar to encourage residents to keep an eye out for more debris. The family members, who covered all their travel expenses themselves, even offered a potential reward to anyone who found a piece of Flight 370.

Grace Nathan, a Malaysian whose mother was on board Flight 370, was among those who made the trip to Madagascar last month. She is deeply frustrated that the families felt compelled to take on the task themselves, and that the underwater search yielded nothing.

"Every single clue to date has been found by private individuals by chance," Nathan said. "Not a single piece of hard evidence has been found by the official search."

Nathan believes the governments' failure to search for coastal debris may have resulted in missed clues.

"They should have done more to initiate something like what we did," she said. "We are laypeople. We don't have the kind of reach they have, we don't have the kind of contacts that they have."

Initially, experts believed that the pieces washing ashore would be virtually useless to the investigation. Too much time had passed, they argued, and ocean currents are too volatile to make it possible to trace the pieces back to their origin.

Yet the recovered parts of the plane provided valuable insight into what happened to it. They confirmed, firstly, that the aircraft went down somewhere in the Indian Ocean. Before the first part — a piece of airplane wing known as a flaperon — was discovered on Reunion Island, farther east of Madagascar, in 2015, some people continued to insist the plane had flown north into Asia, rather than heading south. The flaperon effectively killed that theory and bolstered the investigators' interpretation of satellite data that indicated the plane had ended up somewhere along a vast arc slicing across the Indian Ocean.

A wing flap that washed up in Tanzania also gave investigators clues into what happened in the final moments of the plane's flight. No one knows why the aircraft veered so far off-course after takeoff and turned south into the Indian Ocean, though Malaysian officials have said the plane's erratic movements after takeoff were consistent with deliberate actions. Investigators operated on the theory that the plane was on autopilot in its final hours before it ran out of fuel and plummeted into the sea.

A key question was whether anyone was still at the controls when the plane hit the water, which would affect how far the plane could glide after running out of fuel. Critics who favor the theory that Captain Zaharie Ahmad Shah hijacked the plane say he could have made a controlled ditch at sea in order to minimize debris and help the plane disappear as completely as possible.

But an analysis of the wing flap by Australian investigators suggested it had not been deployed when it hit the water. A pilot attempting a soft landing would have extended the flaps.

The flaperon and other debris also helped officials narrow down the potential search zone. Griffin, the Australian oceanographer, set replicas of the flaperon adrift in the ocean, measuring how fast they traveled and noting how much the wind influences their rate of speed. He and his team then ran computer simulations of how the aircraft parts could have drifted, giving them an idea of where they originated. Late last year, investigators coupled that drift information with a fresh analysis of other data on the plane's movements and concluded that search crews were looking in the wrong part of the Indian Ocean. The investigators recommended crews instead search an area to the north, where they now believe the plane may lie.

But Australia, Malaysia and China have nixed that idea, saying they won't relaunch the search until they have credible evidence pointing to the plane's exact location.

That decision has stunned Blaine Gibson, the American sleuth who conducted a self-funded hunt for the debris along shorelines in the west Indian Ocean. To him, the debris that has been found is the most credible evidence investigators have.

"Debris is the key. Debris is the main clue. Those are actually pieces of the plane," he says. "The debris is really narrowing it down better than it's ever been narrowed. ... There's no excuse for them to not go search that area."