(Releads with source on possible criminal intent)
By Yann Le Guernigou and François Revilla
PARIS/MARSEILLES, France, July 14 (Reuters) - Criminal intent is suspected in two fires which broke out at a petrochemical facility near Marseilles airport on Tuesday as France celebrated its Bastille Day national holiday, a source close to the French government said.
The fires came after an incident last month in which a suspected Islamist beheaded his boss and tried to blow up an industrial gas plant in the suburbs of the southeast city of Lyon. There was however no indication of a link between the two.
"The simultaneous explosion of the tanks, which are spaced about 500 metres (yards) from each other, is not the result of a technical accident, the source said of the incident at a plant run by New York-listed LyondellBasell industries.
"The thesis of criminal intent is clearly being considered," the source added.
Marseilles authorities said no one had been injured and a company spokesman said both fires, which earlier triggered a vast smoke plume in the sky of France's second largest city, had been extinguished by midday.
"The facility is running under normal conditions," the spokesman said.
Another spokesperson refused to communicate output figures but said LyondellBasell employed around 1,000 permanent staff and around the same number of contractors in the facility located in the town of Berre-l'Etang.
On its website, the company describes the "Berre petrochemical cluster" as "one of the largest petrochemical complexes in the south of France."
France is on top security alert after the Lyon attack, which itself came five months after 17 people were killed in Paris by Islamist militants who targeted a satirical weekly and a Jewish foodstore in the French capital.
The fires came a week after explosives, detonators and 40 grenades were stolen from an army base in the nearby town of Miramas. So far, however, there was no evidence of a link between them and the thefts.
Lyondellbasell said the fires broke out around 3:00 am local (0100 GMT) in petrol containers. It said it was doing all it could to limit the environmental damage to the vicinity.
The local Bouches-du-Rhône department said the smoke plume did not pose immediate health risks and that air traffic at nearby Marseilles Provence airport was unaffected.
Michel Cadot, prefect of the local Bouches-du-Rhône authority, told French radio an investigation was underway. (Additional reporting and writing by Matthias Blamont; editing by Mark John)