The National Transportation Safety Board has issued its final report on the deadly KOMO Air 4 helicopter crash in March 2014.
Investigators could not say for certain what caused the crash. But the NTSB report pinpoints a likely scenario, stating that the probable cause of the accident was a loss of hydraulic boost to tail components at take-off, followed by a loss of hydraulic boost to the main rotor controls after take-off.
Investigators couldn't give a specific reason for the loss of hydraulic boost because of heavy fire damage to the hydraulic system and because there was no flight recording device on the chopper.
The report does talk about different possibilities in how the pilot, Gary Pfitzner, may have performed hydraulic systems checks.
He had been trained on the most recent pre-flight check list, but the list inside the chopper was a previous version.