Investigators think a small drone is responsible for causing the emergency landing of a news helicopter flying over Los Angeles last year — the latest sign of the growing risks of millions of civilian drones operating across the U.S.

A National Transportation Safety Board report on the incident noted five previously documented collisions in which drones were fingered as the cause, including one that occurred with a tour helicopter on the Na Pali Coast in 2018.

None of the cases led to injuries, but the small devices have hit with enough force to pierce metal structures, according to investigators.

On Kauai the pilot of a Blue Hawaiian Airbus helicopter reported striking a small unmanned aircraft while conducting an air tour near Nualolo Kai on the Na Pali Coast at 2,900 feet, according to the NTSB.

There were no injuries to the pilot or six passengers, and the drone left only three small scratch marks on the right-side door of the helicopter.

The pilot reported flying above a popular hiking trail when he saw a drone to his left. He turned to avoid it, but apparently another drone struck the right side of the craft.

The helicopter was handling normally, and he continued the tour flight with no incident, the report said.

The latest collision occurred at about 1,100 feet near Los Angeles City Hall on the evening of Dec. 4, prompting the KABC-TV pilot to land after hearing a loud noise, according to the report on the incident.

The object left a hole in the helicopter’s tail structure, “a small gouge” punched into one of its blades, and dented and scuffed the rear of the craft, according to the June 29 report.

Though a drone in the Los Angeles case was never found, evidence in the chopper’s damaged areas pointed to such a device. Tests revealed the presence of polycarbonate, a material used to make “an exemplar popular drone,” around the area of impact.

The damaged areas were consistent with the shape of a small drone, and a computerized reconstruction of the impact by investigators predicted damage similar to what was found. There also was no biological evidence of bird remains.

“Although no drone was located, preventing complete certainty, all the available evidence was consistent with a collision with a small UAS,” the NTSB said in its report, using the acronym for unmanned aircraft system.

A physical search didn’t find a downed drone in the area at the time, the report said. It additionally notes that two drone detection companies in the vicinity said they “did not have any targets in the area at the time,” although both companies “reported less than complete coverage of the area.”

The helicopter’s pilot, who’d logged more than 3,700 hours flying at the time of the collision, said he didn’t see a drone while operating the chopper, according to the report.

KABC reporter Chris Cristi, who was aboard the helicopter when it was struck, told “Good Morning America” that the occupants initially assumed they’d hit a bird, but realized something else was the case upon landing. “It was a scary couple of minutes, and we were all looking at each other, you know, looking for a spot to land, and we eventually put it down and fortunately, everyone’s OK,” Cristi said in the interview.

The Federal Aviation Administration generally forbids small-drone users from flying the devices beyond their line of sight and no more than 400 feet above them.

The FAA is finalizing regulations that would for the first time require civilian drones to broadcast their identity and location, a measure designed in part to lower the risks of midair collisions.

Through June 2019 there had been more than 8,700 reports received by the FAA of drones flying in prohibited areas or in an unsafe manner. There are typically more than 200 such incidents a month when the weather is warmer, according to FAA data.

The first midair collision involving a drone occurred Sept. 21, 2017. The small quad copter operated by a civilian in Brooklyn, N.Y., crashed into an Army helicopter 2.5 miles away from him during a time when the United Nations General Assembly was meeting and flights in the area by civilian aircraft were prohibited, the NTSB found.