Several of 15 Steller sea lions found dead last week on a remote Alaska beach were "deliberately killed," the National Oceanic and Atmospheric Administration said Thursday.

Julie Speegle, a NOAA spokeswoman in Juneau, wouldn't say how the sea lions were killed but said at least some of the deaths were "human-caused."

"We have not seen an incident that involved this many animals at once, probably, for decades," Speegle said.

NOAA Fisheries received a report of the dead sea lions on June 1. Speegle said the endangered animals' bodies were near Softuk Bar, about 45 miles southeast of Cordova.

On June 2, NOAA biologists and enforcement agents, with help from a Coast Guard helicopter crew, visited the site and found 15 dead Steller sea lions on the beach in various states of decomposition. NOAA officials took samples from all the animals and performed necropsies on some, Speegle said.

Three to five of the sea lions had wounds indicating they had been "deliberately killed," Speegle said. It appeared that nothing was harvested from the sea lions and no nets were found, she said.

The Steller sea lions that died near Cordova came from the western stock of the population. The sea lions are divided into two populations at a line just east of Prince William Sound. Between 1976 and 1990, the western stock declined by 75 percent and dropped another 40 percent by 2000, according to NOAA.

In 1997, the western stock was declared endangered. The killing of sea lions in that population is illegal under both the Endangered Species Act and the Marine Mammal Protection Act, with exceptions for Native subsistence hunts.