Norovirus, a winter vomiting bug, struck the care home in Ljungby a week ago, initially causing the deaths of two people, with another guest dying later in the week.
Sweden's national food agency (Livsmedelsverket) said in a statement on Monday that tests had confirmed that imported frozen fruits were the source of the sickness outbreak.
It is suspected that the raspberries became contaminated during the packing process, when boxes of berries were hand-packed by a worker who was carrying the virus.
"It is not uncommon for imported frozen raspberries to be contaminated with Norovirus," Livsmedelsverket said in a statement on its website.
Norovirus can have a greater impact on elderly patients who may already have weaker immune systems than younger people who contract the virus.
Mats Lindbland, an infection control coordinator at Livsmedelsverket told Metro newspaper on Monday: "This confirms the importance of our advice to boil raspberries before serving them. And it is valid especially when serving frozen imported raspberries in retirement homes or hospitals, where those who are eating them may suffer severe symptoms or even die if they are already sick from other diseases before."
Last week it was confirmed that staff at the care home in Ljungby did not boil the fruits served in last Sunday's dessert, despite written guidance on the packaging and the municipality's own safety guidelines.
Officials have since tightened procedures.
"From our side as the supervisory authority, we continue to review… [our] own control measures," said food inspector Josef Larsson.
All those initially infected by the raspberries have now recovered, although some pensioners who were later infected by their peers remain sick.