DALLAS (AP) — Aviation experts expressed doubt Wednesday that turbulence could have caused the deadly February crash of a cargo plane in Texas, suspecting human error or a massive malfunction as more likely culprits.

The National Transportation Safety Board is investigating why Atlas Air Flight 3591 suddenly dropped during its approach to a Houston airport and has not issued a cause of the crash.

The Boeing 767 struck Trinity Bay while traveling at hundreds of miles an hour — an impact that killed the three men aboard and sprayed debris through the swampy area 40 miles (64 kilometers) east of George Bush Intercontinental Airport.

The bay’s deep mud slowed investigators, and aviation safety specialists see the details already released as too little to solve the mysteries of the flight’s final moments. But they think it’s enough to rule out choppy air as a likely cause of the crash.

“Airplanes operate in situations like that all the time,” John Cox, an accident investigator and retired pilot, said of turbulence the plane hit a minute before it entered its fatal drop.

NTSB data show turbulence hasn’t caused a fatal accident on an American cargo or passenger jet in the last decade. Cox and others couldn’t recall a large plane being downed by rough weather since the 1960s.

Rather than precipitation the Atlas Air pilots had maneuvered to avoid, experts said NTSB investigators are likely focused on three events in the plane’s final moments: an engine surge, a small drift up and sharp turn down.

These events are being scrutinized as countries around the world are grounding a different model of Boeing aircraft after two were involved in fatal crashes less than five months apart. On Wednesday, the United States and Canada joined some 40 other nations in ordering all Boeing 737 Max jets grounded amid suspicions about a new automated anti-stall system.

Beyond brand, however, there appear to be limited links between the crashes.

The 767 that crashed in Texas is much older than the 737 Maxs that are being grounded. And no Atlas Air planes are equipped with the anti-stall system that’s come into question, a company spokeswoman told The Associated Press.

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