The Washington Post reported the federal investigators found the Salmonella saintpaul strain in irrigation water and serrano peppers on a Mexican farm where jalapeno peppers are also grown.

"We have a smoking gun it appears," said Lonnie King, a director at the federal Centers for Disease Control and Prevention.

David Acheson, a senior FDA food safety official, revealed the breakthrough Wednesday while testifying at a congressional hearing on the salmonella outbreak.

The U.S. Food and Drug Administration is now warning consumers to avoid raw serrano peppers grown and packed in Mexico.

Since April, more than 1,300 people have fallen ill from Salmonella saintpaul in 43 states, the District of Columbia and Canada, the newspaper reported.