Federal investigators are calling on U.S. and Canadian aviation regulators to require the widespread “immediate” inspection of Bell 407 helicopters as they continue to look into what caused one of those choppers to crash during a tour earlier this year on the Big Island.

Specifically, the National Transportation Safety Board is concerned about the hardware that attaches the Bell 407’s tail boom to its main fuselage. The boom in the June 8 crash, near South Point, was found nearly 800 feet from the main wreckage — and it was missing one of the four bolt-and-washer sets used to attach it, according to a Thursday NTSB press release.

“Our investigation into this accident is ongoing. However, given the findings thus far during the investigation we are concerned that there may be additional Bell 407 helicopters with missing or fractured tail boom attachment hardware,” the crash investigation agency’s release stated.

No fatalities occurred when the Paradise Helicopters air tour crashed into a lava field, but all six people aboard were injured — three of them seriously. One of the passengers said she saw something fall off the helicopter as it spun uncontrollably but wasn’t able to identify what part it was, according to the NTSB.

The NTSB said it still hasn’t determined why the bolt-and-washer set separated. However, that particular set, located on the boom’s upper-left corner, has the “highest tension loading” of the four sets, the agency stated in its release. The other three sets showed signs of fatigue and overload, it added.

Thus, the NTSB is recommending that Bell 407 operators be required to do immediate “torque checks” and visual inspections of the boom attachments. Normally, those checks are required to happen after every 300 hours of flight, it stated. The Paradise Bell 407’s boom separated 114 hours after its last torque check, it added.

“Determining a more conservative interval for the torque check and visual inspection can be challenging given the limited information available,” the NTSB stated.

The investigation into the Hawaii crash is ongoing.