A Boeing 767 appears to have hit turbulence a minute before it entered a steep descent that ended when the plane smashed into a Texas bay in February, killing all three people aboard , according to federal authorities.

“Small vertical accelerations” suggest Atlas Air Flight 3591 entered turbulence soon after the pilots had descended to avoid a band of precipitation as they approached a Houston airport, the National Transportation Safety Board said in a preliminary report released Tuesday.

Seconds after leveling off around 6,200 feet, the cargo plane’s engines surged to “maximum thrust” and it briefly pointed its nose 4 degrees up, according to flight data. The jet then rapidly swung to point 49 degrees downward and began its drop toward the muddy bay, the federal agency said.

An NTSB spokesman, Keith Holloway, said it is still investigating the underlying cause of the sharp change in pitch.

It’s a move that alarmed aviation experts.

“Obviously, going 49 degrees nose down is beyond a radical move,” said Todd Curtis, a former Boeing safety engineer who runs AirSafe.com. “That’s not something an airplane should be doing, especially at that altitude.”

The new details about the plane’s sharp drop were released 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. The U.S. Federal Aviation Administration, however, has stood by the airworthiness of Boeing’s 737 Max following the weekend crash of an Ethiopian Airlines flight.

Curtis said it remains unclear whether a problem with the plane’s systems or human action was primarily to blame for the February crash. If it is a technical issue, he said, that could warrant a broader review of the widely used Boeing 767.

The NTSB previously said cockpit audio suggests the pilots lost control while passing over Trinity Bay, about 40 miles (64 kilometers) east of George Bush Intercontinental Airport.

Before the crash, the plane’s stick shaker, which warns of an imminent engine stall, did not activate, according to the NTSB. That means it’s unlikely the pilots pointed the nose down to avoid stalling.

As the plane dropped, the agency said, it accelerated to 495 mph (797 kph) and gradually pulled up to a 20-degree descent. Curtis said this suggests the crew were trying to pull out of steep fall.

The jet, which had been carrying cargo from Miami for Amazon and the US Postal Service, disintegrated upon impact with the shallow bay.

The NTSB said investigators found one of the plane’s engines and some landing gear west of the rest of the debris, which was spread over 350 yards of the swampy area.

Tidal waters carried some parts of the plane and much of its cargo south, and some wreckage was recovered up to 20 miles (32 kilometers) from the crash site, according to the NTSB.

(AP)