The pilots of a Los Angeles County Sheriff’s helicopter that crashed near the San Gabriel reservoir in March had discussed the possibility of a hard landing caused by blowing dust or contact with nearby trees when they decided to land on a highway turnout, according to a preliminary report by the National Transportation Safety Board.

The helicopter went down on the evening of March 19 near East Fork Road and Highway 39.

Shortly after the helicopter crashed, Sheriff Alex Villanueva held a press conference and said the helicopter crew was assisting the Los Angeles County Fire Department with transporting a patient from a vehicle rollover.

According to the NTSB, the pilot-in-command told investigators they had selected a landing zone on the highway turnout while responding to the emergency call.

The helicopter’s pilots and the helicopter’s crew chief had discussed the risk of reduced visibility due to dust being swept up by the rotors of the helicopter, also known as a brownout, as well as the possibility of hitting trees while attempting to make the landing.

Ultimately, the pilots opted to land anyway but at about five feet above ground the crew chief called for pilots to “hold.” The helicopter then experienced a brownout as dust swirled up and reduced visibility and then the aircraft made unexpected contact with a tree, NTSB officials said.

The helicopter touched down, rolled onto its left side and came to rest near a tree.

All occupants of the helicopter had to be assisted by first responders. Both pilots, as well as two passengers, were seriously injured. Two other passengers suffered minor injuries.

During the rollover, the pilot-in-command’s lap belt separated from the seat. Investigators later determined that the brackets used to keep the pilot safely secured in the helicopter had fractured, the NTSB said.

The helicopter’s fuselage and main rotor sustained major damage but all major components were located at the scene of the crash.

The crash remains under investigation by the NTSB. Investigations of this nature can typically take several months to a year to complete, officials said.