ASTANA -- The death toll in a sudden die-off of critically endangered saiga antelope is nearing 20,000, according to authorities in northern Kazakhstan who are still unsure of the cause.

The Kazakh Department for Emergency Situations said that 19,231 carcasses had been buried in the Qostanai region by late on May 19.

Officials in the adjoining Aqtobe region also reported on May 20 that they had found 68 dead saigas in the Irgiz-Torghai natural reserve.

The cause of death is unclear, but authorities suspect the culprit is a bacterial infection carried in the mouth and breathing passage known as pasteurellosis.

Kazakhstan is the primary habitat for the saiga, easily recognizable with its distinctive big, bulging eyes, tubular snout, and spiraled horn, which is used in traditional Chinese medicine.

Its numbers, once in the millions, were severely depleted after the Soviet breakup by hunters and poachers eager to bag the odd-looking steppe dweller for trophy, meat, or the sale of its horns.

Regional authorities in Qostanai have declared an emergency as they struggle to dispose of the dead saigas and search the steppe for more carcasses.

The regional governor and senior Kazakh Agriculture Ministry officials visited the area this week to coordinate the response to the biggest die-off of the species in recent memory.

Kazakh officials initially said last week that they had found more than 100 dead animals, then raised the death toll to 1,000 on May 13.

Officials say the carcasses bear no wounds or other signs of trauma.

The total population was thought to have fallen as low as around 21,000 in 2003, when the species was declared critically endangered.

More recently, groups like the Saiga Conservation Alliance report conservation efforts appeared to be working, with the animals' numbers possibly in the hundreds of thousands.

Deputy Agriculture Minister Erlan Nysanbaev said last week that experts had been collecting samples of soil, water, and the flesh of dead animals and sending them to a state laboratory in the capital, Astana. He said preliminary test results were expected this week.

There have been sizable die-offs of saiga in the past decade in Kazakhstan, as well as instances in which large numbers of the animals were apparently killed by poachers for their horns, which fetch around $80 a pair on the Kazakh black market.

Activists blamed a 2012 discovery of nearly 1,000 dead saigas, also in the Qostanai region, on launches at the Baikonur Cosmodrome, which Russia uses for its space programs. Authorities said then that the cause was probably pasteurellosis, but that was never officially confirmed.

Several Russian Proton-M rockets, which use highly toxic fuel, have exploded over Kazakhstan after launch from Baikonur, sparking concerns among local residents and environmental activists.

Most surviving saiga populations are found in Kazakhstan, Mongolia, and Russia's Kalmykia Republic.