Authorities in Yichang, Hubei province, said on Monday that they are still investigating the source of a number of dead pigs spotted in the Yangtze River last week.

Local authorities have been removing carcasses from the river since Saturday, but have not been able to identify which farm they came from or why they were dumped in the river.

So far, four pigs have been removed from the water and their bodies burned to prevent the spread of disease, said Dan Yazi, a spokeswoman for the Yichang publicity office.