The accident was “without doubt” due to negligence or error, Environment Minister Delphine Batho says.
The two investigations – one judicial, the other administrative – will look into whether people’s lives or health have been endangered.
“This is an utterly abnormal situation that could have been avoided,” said Batho, who flew back from celebrations of Franco-German friendship to oversee the handling of the accident.
The chemical released, mercaptan, is added to gas to enable the detection of leaks
After first assurances that it is not toxic, officials now say that it can be harmful in heavy doses but insist that the amounts released were not sufficient to be so.
The stench, compared to the smell of sweat, garlic and rotten eggs, reached as far as southern England and sparked thousands of calls to emergency services there and in France, where it could be detected in Paris.
An operation to end the pollution started at 1.00am on Tuesday night, local officials said Wednesday, allowing 260 employees in the five factories nearby to return to work.
Two out of 36 tonnes of the solution had been treated on Wednesday morning and the operation was set to last several days.
It was better “to take time than to take risks”, Batho said.
Only half of the 421 chemical plants in France that are legally required to have risk prevention plans have them, she told Europe 1 radio.