Extensive tests have to failed determine the source of salmonella found in alfalfa products, which affected 120 people across Perth.

Western Australia's Health Department launched an investigation into the Parilla Fresh factory in Hamilton Hill last month. The factory was forced to halt production as alfalfa products were recalled. But despite tests on the factory's seeds, production process and workers, investigators have found no sign of salmonella.

Health Department spokesman Paul Van Buynder says authorities will still monitor the factory's production, which is expected to restart in a few weeks.