A mid-summer Salmonella outbreak that sickened at least 124 people across the country probably was due to contaminated lettuce.

At least that’s the opinion of William Keene, senior epidemiologist at Oregon’s Public Health Division.

The outbreak ran from mid-July to mid-August with one cluster of cases being in the Portland area where seven became sick and two required hospital stays. They have since been released.

No deaths were associated with the outbreak.

Keene said shredded lettuce is suspected of causing the outbreak and the investigation by the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) continues.

Salmonella Typhimurium was involved, which makes the investigation more difficult because the strain is so common. Nor were there any lettuce recalls associated with the outbreak, and by now all tainted product would be gone.

“We’re trying to learn what happened and what steps can be taken to reduce risk,” said Keene.