(CNN)"Good night Malaysian three-seven-zero." It was a routine sign-off, an all-is-well. On March 8, 2014, at 1:19 a.m., someone spoke those last words from Malaysia Airlines Flight 370 to air traffic controllers before the Boeing 777 vanished. A year later, searchers have no new clues as to where it went with 239 people on board. Radar and satellite reports have provided hints, but searchers still have nothing to hold in their hands. No wreckage seen floating at sea or beached on shore. No fuselage resting on the sea floor. Experts have said the data indicate the flight path from Kuala Lumpur, Malaysia, to Beijing unexpectedly veered, putting the commercial jet over the southern Indian Ocean. But the water's vast and intricate depths have revealed no secrets. And as clarity has eluded grasp, analysts have made many speculations about what happened. The most controversial idea: Is the maritime search area all wrong? Did the plane land clandestinely on solid ground? Here are some expert theories about what happened to MH370. Investigators have since cast doubt on some of their details. Who radioed those last words to air traffic control? Capt. Zaharie Ahmad Shah? First officer Fariq Abdul Hamid? There was much speculation over that, but the Malaysian inspectors in April said it was Zaharie. The pilots were supposed to check in with new air traffic controllers in Vietnam, but never did. One theory is that one pilot may have incapacitated the other, then guided the plane to its end, taking the passengers down with him in a dramatic suicide. Mark Weiss, a retired American Airlines captain, has flown a Boeing 777. He believed shortly after the crash that there may have been a struggle. "It was one of the pilots that maybe had a meltdown or did something nefarious to the airplane," he said. But Malaysia Airlines Chief Executive Ahmad Jauhari Yahya has vehemently defended his employees, particularly the pilot. "We do not suspect any one of our crew until there's such evidence. ... Captain Zaharie is a very capable man," he said. "We have no reason to suspect (him)." Weiss also thought there could have been another person -- a crew member or someone else -- in the cockpit who "was bent on perhaps committing suicide or doing some destruction on the aircraft." Copilot Hamid, 27, reportedly once invited a woman and her friend into the cockpit on a 2011 flight between Thailand and Malaysia. "That's an enormous breach of security," Weiss said. But investigators punched holes in the idea, saying there was no indication of a third party in the cockpit, CNN reported in April. The difference between hijacking and commandeering is nuanced. The former term is often used when the hijacker issues a demand such as being taken to a safe-haven country or receiving ransom to release passengers. When people commandeer a plane, they might keep the motives secret, said political analyst Peter Bergen. They may want to steer it themselves at a target, like the September 11, 2001, terrorists did. In 1994, a FedEx employee burst into the cockpit of FedEx Flight 705 with a hammer and spear gun. He wanted to crash the plane into the company's Memphis, Tennessee, headquarters. The crew thwarted that takeover. "Commandeering would fit with the few facts that we do know, and (it's) certainly a theory that we haven't heard a lot of that isn't a conspiracy," Bergen said. Experts are divided on this theory, partly because no terrorists have claimed responsibility at a moment when they would have the world's attention -- unless potential terrorists were waiting for something. MH370 went to Kazakhstan. Outlandish conjecture or genius insight? The theory that Russian actors on board MH370 found a way to get the plane through the border territory of China, Pakistan and India to a Kazakh landing strip leased to Russia comes from science journalist and private pilot Jeff Wise. Fleets of ships and search aircraft are looking in the wrong direction, he says. The airliner went north, not south. Investigators may have misinterpreted a key component of the Inmarsat satellite data. "This is not a normal investigation. They need to throw out the book," Wise has said. Another aviation analyst, David Soucie, also cast doubt after MH370 went missing on the most widely held belief that the plane hit the Indian Ocean. "If it had crashed in the way that we think it did, which is to run out of fuel and hit the water and break up into pieces, there would be pieces somewhere," he said. But Michael Exner, an engineer with decades of experience in the mobile satellite communications industry, says the data "accurately and unambiguously" shows MH370 went down near the so-called 7th arc, a path along which the search has been focused. "The current ATSB search strategy remains the best search strategy," he said. In a less sinister but equally lethal explanation, some experts theorized the plane mysteriously crashed somewhere because of mechanical malfunction. Perhaps the electronics died, or a fire broke out, preventing the pilots from communicating. Maybe they turned to look for a landing strip but couldn't steer the plane properly. Pilots had trouble embracing the thought. "I've been running that in my brain now ever since this thing happened," said Jim Tilmon, an aviation expert and retired American Airlines pilot. "One possibility would be a total electrical failure which is very, very hard to imagine because (the plane) has so many generators coming from different places," Tilmon said. If they fail, there are other backups. He has said he's never heard of anything like it happening before. For months after MH370 disappeared, Malaysian officials reported details of the search to next of kin and the public. Something would be spotted, hopes rose, and then it didn't pan out. Hopes were dashed; anguish returned. Sunday, a year to the day after the plane disappeared, the international independent investigation committee released an interim report on the disappearance of MH370. It found no indications of unusual behavior among the aircraft's pilots and cabin crew before it took off, and said that Zaharie had had no personal or financial problems that would cast suspicion on him. The interim report also revealed that the battery of the underwater locator beacon on the plane's flight data recorder had expired more than a year before its disappearance. The battery on the plane's other so-called black box, the cockpit voice recorder, had been replaced as scheduled, and remained within its expiry date, the report said. CNN's Pamela Boykoff and Jethro Mullen contributed to this report.
Many experts believe the plane went down in the South Indian Ocean . The most controversial theory: Russians hijacked the plane to Kazakhstan . Electrical failure is another theory .