Poor aircraft maintenance, including lack of grease in a vital drive shaft, was pegged as the probable cause of a helicopter crash near the USS Arizona Memorial that killed a 16-year-old boy in 2016, according to the National Transportation Safety Board.

The Bell 206B helicopter owned by Genesis Helicopters was carrying a pilot and four visitors from Canada when it crashed Feb. 18, 2016, about 20 feet from the shoreline. The helicopter sank in 40 feet of water. Riley Dobson of Ontario became tangled in his seat belt and suffered brain injury while submerged. He died four days later. His parents, another family member and the pilot survived.

The pilot told the NTSB that shortly before the crash he noticed a vibration in the cabin that grew into a grinding sensation, according to the final NTSB report released last week. A warning light indicated the main rotor was turning at a low rate, while the engine began to rev up. The pilot said he tried to land as close to shore as possible to avoid sightseers at the memorial when the helicopter lost lift about 20 feet above water and “fell out of the sky.”

During the investigation, the owner could not produce mandatory records for a current annual inspection, a current 100-hour inspection or the maintenance on the failed drive shaft, according to the NTSB report. Several items on an inspection checklist also were overdue, the agency said.

As for the drive shaft, it likely overheated due to a lack of grease, causing components to shift and wearing down the gear teeth, the report said. When the drive shaft failed, the engine would not have been able to drive the main rotor.

Since the crash, the company appears to have closed, according to state business records, and its owner, Jeffery Gebhard, could not be reached for comment.

The NTSB report said increased inspections by the Federal Aviation Administration might have uncovered the company’s poor maintenance and record keeping.