An unusual mass stranding of pilot whales in Hawaii, first reported early Friday morning at a beach in Nawiliwili Bay, Kauai, resulted in five dead by early evening — with the possibility of a higher toll.

The cause of the stranding might take weeks to determine but could relate either to natural or human-related factors, scientists said.

Necropsies began Friday afternoon in concert with the University of Hawaii’s Hawaii Institute of Marine Biology, but laboratory results from tissue samples could take weeks to analyze.

While whale mass strandings are most common among pilot whales — and happen with large groups along the East Coast and in New Zealand — mass strandings of any species of whale are unusual in Hawaii, said David Schofield, marine mammal response coordinator for the National Oceanic and Atmospheric Administration.

The most recent mass stranding of whales in Hawaii involved melon-­headed whales in 2004, Schofield said.

Scientist Robin Baird of the Cascadia Research Institute, based in Olympia, Wash., said there are communities of pilot whales living around the main Hawaiian Islands, including one off Kauai. There are also resident communities from Hawaii island, Niihau and West Oahu, while other communities live in the open ocean, he said.

Baird has tagged more than 100 pilot whales over the last 18 years in Hawaii to determine the effects of sonar during Navy training. To detect prey and determine their depth, these whales use midlevel sonar — the same frequency range as the Navy, he said.

Baird said much of their work, with some funding from the Navy, is done off Kauai, home to the Pacific Missile Range Facility. The Navy is mandated through the Marine Mammal Protection Act to try to understand how their activities affect the whales and dolphins, he said.

“A lot of their efforts are concentrated off Kauai,” he said.

A 2016 study of a resident Kauai community of pilot whales revealed that they spent less time foraging during the Navy exercises but did not leave. Baird interprets that as disruption of behavior, irritating and possibly painful, depending on the proximity of the exercises.

The Navy said no major exercises were being conducted in the area at the time of the stranding, and it had no indication Friday of any use of sonar.

“The stranding of a marine mammal is an unfortunate but routine occurrence in nature,” the Navy said in a statement. “The Navy employs protective measures to minimize the potential for its training and testing activities to harm the marine environment.”

Navy spokesman Bill Doughty added, “We are doing a thorough review of what was in the area,” pulling training records, and results of that review will be available Monday.

Baird, who has a catalog of pilot whale photos, is asking for any photos from the public of the pilot whales before or after going into Nawiliwili Bay.

“It’s more likely they just came into the area within 24 or 48 hours,” he said.

A necropsy could reveal gas bubble lesions in some cetacean species. If the gas is tested, it could show whether the animal suffered decompression sickness, which might indicate it was exposed to sonar.

Navy sonar is loudest, but oceanographic mapping sonar has also been associated with strandings, he said.

Paul Nachtigall, professor emeritus at the marine biology institute, said pilot whales “have beautiful, big round foreheads filled with fats and oils, and they make a very loud sound inside their head.” He said they use a pencil-like beam of sound to bounce off fish or squid to locate a meal.

He said a necropsy will not show whether the animal died due to sound exposure since there is no physical effect on the ear unless it is a loud explosion.

He said in Japan, fishermen hit pipes with a hammer to drive whales to shore, but that is not nearly as loud as Navy sonar. The sounds don’t harm the animals, he said.

“It just sort of frightens them and scares them, and they flee.”

He said pilot whales are long-lived and have strong social bonds. If a longtime matriarch gets sick and beaches herself, the others might follow.

Females live into their 60s and 70s; males, into their 40s and 50s.

“They’re air-breathing animals, and sometimes they come ashore to die,” Nachtigall said.

An off-duty Coast Guard member spotted the whales while surfing and reported them to the Coast Guard station on Kauai, which notified NOAA.

NOAA officials responded at 6:30 a.m. Friday and discovered two dead and five stranded at Kalapaki Beach, with likely others nearby.

Coast Guard and Ocean Safety personnel, Kauai firefighters, NOAA staff and volunteers, and canoe paddlers refloated the five and herded them out to sea, said Schofield. But there was a chance they could return within a day or two, he said.

A spokesman for the Department of Land and Natural Resources confirmed Friday evening that three more had been found dead.

NOAA worked with community Hawaiian cultural practitioners, who have deep spiritual beliefs about whales, and they attended the necropsies, Schofield said. The carcasses will be buried on land, he said.