French authorities defused an explosive device at a European sport rehabilitation centre on Thursday in the southwest of the country, prosecutors said, AFP reported.
The device was made of flammable liquid inside of a water bottle, with an igniter attached, prosecutors in Paris said.
A worker at the research centre in Capbreton discovered the explosive around midday and alerted authorities.
"Everyone was evacuated and a security perimeter was put in place while bomb-disposal experts worked," a source close to the investigation said, describing the device as rudimentary.
No group had claimed responsibility for the explosive and investigators could not yet say whether it involved "terrorism," the source said.
Police specialists were dispatched and anti-terrorism prosecutors based in Paris were also to be involved in the investigation.
The centre is a complex of buildings located on a boulevard along the waterfront.
It treats athletes after they suffer injuries, including Brazilian footballer Ronaldo and Formula One driver Michael Schumacher, said Stephane Delpeyrat, a local government official.
"It is the most recognised centre of its type in Europe," he said.
Earlier this month, an unknown group placed dynamite in a Paris department store and threatened more attacks if French troops were not pulled out of Afghanistan.
The dynamite was made safe by police after they had evacuated the shop and closed off surrounding streets.