FAIR OAKS, Calif. -- KCRA 3 Investigates has learned of a potential safety problem with the model of helicopter that crashed near Lake Natoma Wednesday night, killing two sheriff's deputies and critically injuring a third.

The Federal Aviation Administration issued an airworthiness directive effective last month. It says the airworthiness authority in France, where the model of helicopter is manufactured -- told the FAA that an unsafe condition may exist on the helicopters.

The FAA directive requires inspections of the tail rotor drive shaft damper half- clamps. It says that "if not correctly positioned," it could result in "failure of the drive shaft and subsequent loss of control of the helicopter."

While FAA directives are routine, helicopter experts told KCRA 3's Lynsey Paulo that they are also significant, and that compliance is mandatory.

According to the FAA, there are 75 EC-120B helicopters registered in the United States, with 17 of them in California. Many of the helicopters, which are made by American Eurocopter, are owned and-or operated by law enforcement agencies. The Sacramento County Sheriff's Department has three, including the one that crashed.

The pilot, Joseph Kievernagel, a 17-year veteran, and co-pilot, Kevin Blount, a seven-year veteran, died in the crash. A third deputy, Eric Henrikson, a seven-year veteran, was taken to Mercy San Juan Medical Center in critical condition.

Pilots that Paulo talked with said the EC-120B helicopters are ideally suited for law enforcement because they have a good safety record, are state of the art, comfortable and quiet and perfect for long patrols. Tour operators and corporations also use them.

The NTSB has reported four accidents in the past five years involving this model helicopter. Two were pilot error, and two are still being investigated.

Wreckage Moved

On Thursday, NTSB investigators fanned out across the hillside where the helicopter came to rest after the crash, looking for clues as to why it dropped from the sky.

It's unknown what caused the helicopter to go down, but witnesses said it appeared to be having mechanical problems.

"We started to see some flames shoot out the back of the helicopter. We were wondering what that was all about when all of a sudden we heard a pop. The helicopter kind of tilted -- the nose came up -- and you could tell that the pilot was trying to control it and land it safely. It was definitely going down," said witness Alfred Todd said.

The wreckage of the crash was placed on a flatbed big rig about 4 p.m. and taken to an impound yard off Highway 99 between Sacramento and Yuba City, where the investigation will continue.

Crash Map | Video: Investigation Begins | Video: LiveCopter 3 Over Scene |
Video: The Crash Victims