Environmentalists had said that toxic, foul-smelling hydrogen sulfide gas emitted by the rotting seaweed had poisoned the animals in the Cotes d'Armor region of Brittany.
Ecologists say that nitrates pollution in rivers from fertilizers used in intensive farming has boosted the growth of algae along France's coastline.
However, tests on the bodies of six wild boars found widely ranging traces of the gas and one carcass showed no signs of the gas at all.
"With such a wide difference in the values, we cannot reach any conclusion," said a spokesman for the local prefecture.
The beach near the town of Morieux where most of the animals were found in late July has been closed to the public. Authorities are considering a range of possibilities for the deaths, which have been front page news in France, such as deliberate poisoning of the animals.
The government launched a plan to fight against the spread of seaweed along the coast in 2009 after an asphyxiated horse found in Cotes-d'Armor was discovered to have a large amount of hydrogen sulfide in its body.