The smoke that filled the cabin of a Delta flight as it took off from the Atlanta airport in February was so thick that the lead flight attendant had trouble seeing past the first row of passengers and the pilots donned oxygen masks as a precaution.

The National Transportation Safety Board said in a preliminary report Wednesday that the plane quickly returned to the airport on the morning of Feb. 24 and evacuated all 99 people aboard. Two people sustained minor injuries during the evacuation, but no one was hurt by the smoke.

Initially, the airline described the incident as just a haze inside the Boeing 717 aircraft. A Delta spokesman said he couldn’t comment during the NTSB’s ongoing investigation that the airline is cooperating with.

The flight attendants reported that the smoke began near one of the doors in the front of the plane before it also started coming out of all the vents throughout the plane, according to the report. The flight attendants tried contacting the pilots but initially couldn’t reach them because they were focused on emergency procedures and flying the plane.

The flight attendants assured passengers they were trained for the situation and asked them to remain calm.

Shortly after the smoke appeared, the NTSB said, the pilots got a low oil pressure alarm for the right engine, so they shut it down as they were returning to the airport. When maintenance personnel inspected that engine after the plane landed they found little or no oil in the engine.

The NTSB hasn’t determined if that oil leak was the cause of the smoke. That won’t be established until the agency completes its full report sometime next year.

The plane was met by firefighters when it landed, and when the pilots opened the flight deck door, they “noticed a tremendous amount of smoke in the cabin, and the captain immediately ordered an evacuation,” the report said.

Passengers evacuated the plane through a combination of the emergency slides at the front and back of the plane and climbing off the wing.

The flight’s destination had been Columbia, South Carolina.