Dozens of jackdaws were found in the centre of Falkoeping prompting comparisons with the mysterious deaths of 3,000 birds in the US.
Sweden's National Veterinary Institute said it had tested five of the birds and found evidence of traumatic injury.
But it could not explain why the birds were on the street.
Professor Marianne Elvander, the institute's head of disease control and epidemiology, told the BBC News website that they appeared to have been hit by some kind of blunt instrument.
"My best guess is that this was the secondary effect, that they could have hit the ground very fast after being hit by a car or truck," she said.
Swedish media have described how Falkoeping resident Drilon Hulaj came across "hundreds of dead birds" lying in the street while driving home on Tuesday night.
There have been no reports of fireworks or storms in Falkoeping on Tuesday night and fireworks has been ruled by vet officers, who say there is no sign of internal haemorrhaging.
But Swedish newspapers have suggested that the birds had been pecking at salt on the road when they were run over by a passing lorry.
Rescue chief Christer Olofsson told Aftonbladet that a lorry driver had called him to explain that he had hit a lot of the birds but had not considered it significant until he discovered the story had attracted international media attention.
While the lorry may go some way to solving the Swedish mystery, vet officers have pointed out that many of the birds were not run over.