US officials hunting the source of a deadly E.coli outbreak have searched two California spinach producers.
Health experts say spinach on US shelves is safe to eat again
Agents from the FBI and the Food and Drug Administration visited the plants searching for evidence of malpractice or negligence.
Contaminated California spinach is thought to be the source of an E.coli outbreak that killed one woman and left some 200 others unwell across the US.
Officials said they did not believe the spinach was deliberately contaminated.
One of the companies searched, Natural Selection Foods, had already recalled its spinach from US stores as infections spread across the country.
The other, Growers Express, had not previously been linked to the outbreak.
Northern California prosecutor Kevin Ryan said investigations were focusing on allegations that "certain spinach growers and distributors may not have taken all necessary or appropriate steps to ensure their spinach was safe".
Officials stressed that spinach now on sale in the US was safe to eat, as all tainted spinach had been recalled.
Based in San Juan Bautista, California, Natural Selection packages spinach sold under 34 brand names.
It stopped spinach production when a virulent strain of the E.coli bacteria - O157:H7 - was found in spinach sold under the Dole brand.
The company's chief executive, Charles Sweat, said Natural Selection would co-operate fully with ongoing investigations, but defended the company's practices.
"All tests performed on our processing facilities, both those done by independent scientists and government investigators, have been negative," he said in a statement.
"We continue to believe that the source of the contamination was in the fields from which we buy our spinach."
Growers Express did not comment on the search.
Infection by the E.coli bacteria causes diarrhoea and possible kidney failure.
Normally found in the intestines of people or cattle, it is particularly harmful to young children and pensioners.