A carcinogenic protein derived from leather has been found in milk products made by a Chinese dairy.

Morning Garden, based in China’s eastern Zhejiang province, added protein powder extracted from hydrolyzed leather waste to several batches of milk drinks, according to the local Lanxi government website.

The protein is thought to have been used for a similar purpose as the industrial chemical melamine, which caused a national scandal last year. At least six infants died, and 300,000 fell ill after consuming melamine-tainted milk powder.

Milk diluted with water but containing melamine appears to have the same protein content as undiluted milk. Several Chinese media have reported that the leather protein is harder to detect in tests than melamine.

This latest food safety incident was first reported to China’s top quality watchdog in February, the Chengdu Business Daily reported. Local authorities carried out tests in March and found the leather protein in five out of eight batches of Morning Garden products.

The toxic material was detected again in new batches earlier this month. Police have detained three company representatives and seized almost 2,000 packs of adulterated dairy drinks.

The Guangzhou-based Nanfang Daily quoted an anonymous source who said “[The use of] leather protein has long been a public secret in the industry. It [Morning Garden] is not the only one who uses it.”