Experts are still trying to find what exactly caused serious eye injuries on dozens of patients nine months after they received eye surgeries at two hospitals, according to China’s top drug regulator.

The China Food and Drug Administration released a statement on Thursday night after media reports that 18 patients became blind in one eye after having surgeries at Peking University Third Hospital, one of the top hospitals in Beijing, in June.

The CFDA said in its statement that 71 patients suffered injuries after surgeries in the hospital and Affiliated Hospital of Nantong University in Nantong, Jiangsu province last year and they are caused by substandard perflutren, a medical gas which is used during surgery as a temporary filler, during the surgeries on these patients.

The two hospitals bought a total of 150 boxes of the gas of the same batch and used most of them in May and June, and the other 13 boxes were sent to local test centers for testing, the CFDA said.

Test results released in July showed that the batch of products were not even in quality and some of them failed standards, but experts could not determine what the impurities were in the gas due to restriction of test technologies currently available and lack of samples, it said.

The CFDA said the same batch of products, produced by Tianjin Jingming New Technological Development Co, in Tianjin, were sold to 25 provinces, municipalities and autonomous regions in China and 621 boxes were used by 82 hospitals in addition to the two hospitals, but no other injuries were reported. The producer recalled all the 8,632 boxes suspected to have problems in July to eliminate risks, it said.

Local authorities in Tianjin imposed a fine of 5.18 million yuan ($798,000) on Tianjin Jingming New Technological Development Co for producing substandard medical apparatus, and confiscated all the perflutren it produced, CFDA said.

The company has suspended production of the gas and local authorities demanded that the company not resume production of the gas until it finds causes of the substandard gas, according to CFDA.