HONG KONG — Thousands of dead fish floating along a 19-mile stretch of a river in Hubei Province in central China were killed by pollutants emitted by a local chemical plant, provincial environmental officials said Wednesday.

Environmental protection officials said tests on water taken from the Fu River upstream from the metropolis of Wuhan revealed that extremely high levels of ammonia in the water were caused by pollution from a plant owned by the Hubei Shuanghuan Science and Technology Company.

The tests, conducted by environmental officials from Xiaogan City, revealed ammonia concentrations downstream from the plant as high as 196 milligrams per liter. The World Health Organization notes that naturally occurring ammonia appears in surface water at concentrations of about 12 milligrams per liter, while the similar figure for drinking water is around 0.02 milligrams per liter.

Shares in Hubei Shuanghuan were suspended from trading on the Shenzhen stock market Wednesday pending the release of an announcement. Calls to the company’s headquarters were not answered on Wednesday. Provincial officials ordered the company’s plant to cease production while the cause of the leak was investigated.