A Hawaiian monk seal killed Thursday on Kaua'i's north shore was a pregnant female who had previously given birth to four pups, according to NOAA Fisheries.

Witness accounts suggest the 600-pound monk seal, known to NOAA researchers as RK06, was shot to death, but federal officials would say only that the endangered marine mammal died as a result of "foul play."

A necropsy on the monk seal was conducted yesterday, and NOAA Fisheries spokeswoman Wende Goo said no further details were being released because the killing is under investigation.

"Foul play" by humans also is blamed for the death of a 4-year-old male monk seal found dead on a Kaua'i beach April 19, according to Goo.

"We don't have enough information to link the two," she said.

The latest monk seal killing is particularly troubling because of the female's breeding history.

"She was an important part of the breeding community," Goo said.

A Kaua'i couple told The Advertiser they arrived at Pila'a Beach at about noon Thursday and were standing on a hillside above the beach when they spotted the female seal resting on the sand. As they began to head down a trail to the beach, they heard what sounded like four gunshots and then saw the monk seal scamper toward the ocean.

The couple, who do not want to be identified to protect their safety, said they saw a man standing next to a white pickup truck but did not see him with a weapon. When they got to the beach, they talked briefly with the man, but did not notice anything unusual.

About 45 minutes later, the couple saw the dead seal rolling in the surf and spotted a "perfect little puncture wound" on the animal's side, about two feet from its rear flippers.

They reported the incident to state Department of Land and Natural Resources officials and NOAA authorities.

The Hawaiian monk seal is one of the world's rarest seal species. They are protected under the Endangered Species Act, the Marine Mammal Protection Act and state wildlife laws.

In recent years, the marine mammals have been seen more frequently around the main Hawaiian Islands, where their population has been estimated at 100. However, their numbers are declining in their favored habitat in the Northwestern Hawaiian Islands, where the monk seals are facing starvation, entanglement in fishing gear, shark predation and disease.

Scientists estimate the monk seal population in the Northwestern Hawaiian Islands has dropped from about 1,400 in 1998 to just over 1,000 in 2007.