Two bags of Dole baby spinach that tested positive for the E. coli strain that has sickened 175 people nationwide were packaged at the same plant on the same day, California health officials told The Associated Press Monday.

That information has helped investigators tracing the source of the tainted greens narrow their search to nine farms in three California counties linked to grower Natural Selection Foods LLC.

The tainted bags, which were found in Utah over the weekend and in New Mexico last week, were processed at Natural Selection Food’s San Juan Bautista plant during the same shift on August 15, 2006. The firm packages spinach under many brand names, including Dole.

It is too soon to say whether any other brands besides Dole would turn out to have been contaminated. Inspectors are focusing exclusively on Natural Selection at this point.