Chinese officials say they have arrested more than 100 people suspected of making "gutter oil," illegal cooking oil made from waste and sold to restaurants.

In a food safety crackdown, 13 underground workshops were shut down and more than 3,584 tons of "gutter oil" were seized, the official Xinhua news agency reported.

"Gutter oil" is made by taking used up cooking oil, often left over from restaurants, along with spent animal fat, and reprocessing it into edible oil to be sold to restaurants.

Investigators found suspects used not only waste fat and oil, but also fat from rotten meat and slaughterhouse waste.