A salmonella outbreak that sickened dozens of people in Washington and 18 other states appears to be over, while investigators remain unsure how it began. Health officials believe the outbreak may be linked to tomatoes or other fresh produce contaminated with the bacteria, but they haven’t been able to pinpoint a specific product or source.

There have been no deaths, but 171 people have fallen sick.

The CDC continues to interview people sickened in the outbreak, but the information gathered is too vague or broad for the FDA to trace the contamination to its source.