Police said Tuesday that two people who alleged poisoned milk and led to the death of three babies in northwest China, did so out of anger against the farmer who produced the milk.

Investigations show that nitrite, an industrial salt that can be poisonous, was intentionally added to milk produced on Ma Wenxuan's dairy farm in Mafang Village, Pingliang City of Gansu Province.

Police allege that a man surnamed Wu and his wife, surnamed Ma, added the chemical to the milk because they were angry with Ma Wenxuan over some unresolved business disputes.

The couple and Ma Wenxuan co-rent the same piece of land and farm dairy cows on it, according to a spokesman with local public security bureau.

Wu bought nitrite around April 1. On April 5, Wu's wife added a little amount of it to Ma Wenxuan's milk. People who drank the poisoned milk displayed only slight symptoms of poisoning.

On April 6, the woman again poisoned Ma Wenxuan's milk but this time with more nitrite.

Thirty-nine people in Pingliang were sent to the hospital after drinking the nitrite-tainted milk. Three children under two years died, the youngest being 36 days old.

The couple are detained pending further investigations, the spokesman said.