A U.S. Coast Guard boat searches the area near Freeland, Washington, on Whidbey Island north of Seattle on September 5, where a chartered floatplane crashed September 4.
CNN  — 

Federal investigators are asking the manufacturer of a seaplane that plunged into a Washington state bay last month to mandate inspections of the plane’s tail section, citing an issue that could potentially lead to a possible loss of airplane control.

The request comes after a float plane carrying 10 people crashed into Mutiny Bay on the afternoon of September 4, killing everyone on board, including a child.

The investigation into the crash by the National Transportation Safety Board found “a possible safety issue with a flight control” on the De Havilland Canada DHC-3 Otter after recovering 85% of the wreckage from the bay, the agency said Monday.

“Investigators found the horizontal stabilizer actuator, which is part of the airplane’s pitch trim control system and is also referred to as the trim jack, had separated into two pieces,” the agency said.

They concluded those pieces separated by unthreading, not by being pulled apart. The safety board also explained that investigators were unable to locate a locking ring that holds the two pieces in place.

“At this time, the NTSB does not know whether the lock ring was installed before the airplane impacted the water or why the lock ring was not present during the airplane examination,” the agency noted, underscoring that it has not reached a final conclusion as to the cause of the crash.

The board, along with Transportation Safety Board of Canada, has asked the maker of the aircraft to draft written instructions on how to conduct inspections of the actuator to ensure that the lock ring is in place and properly engaged to prevent the separation of the two pieces.

“Those instructions will be released and provided to all operators of the DHC-3 airplane worldwide in a Service Letter,” investigators said.

At the time of the crash, Broadway star Megan Hilty said on social media her pregnant sister Lauren Hilty, brother-in-law Ross Mickel and their toddler son, Remy, were aboard the plane.

“My beloved sister Lauren, brother-in-law Ross and baby nephew Remy were on that plane,” Hilty wrote last month on her verified Instagram account. “To make this news even more devastating, my sister was eight months pregnant and we were expecting to welcome baby boy Luca into the world next month.”

The investigation into the crash remains underway as the transportation board works to determine a probable cause for it.

“An evaluation of lock ring failure modes and lock ring installation instructions will be accomplished in addition to an aircraft performance study,” the board said.

CNN’s Lisa Respers France contributed to this report.