Daily newspaper Le Figaro writes that the group, calling itself AZF made the threats against eight multinational companies in July, but officials have found no evidence of a pending poison plot, said the national police spokesman Patrick Hamon.
"The goal is to identify and apprehend the blackmailers," he added.
The Le Figaro article refers to two separate news pieces in the local French papers Aujourd'hui en France and La Dépêche du Midi that reported more than 30 post offices in the Toulouse area had been under police surveillance during last week.
On Monday Le Figaro printed a copy of the letter that the newspaper received last month, featuring the letterhead 'AZF', the name of a chemical factory that exploded in the southwestern city of Toulouse in September 2001, killing 30 people.
The police spokesperson said the threats appear unrelated to blackmail attempts, shortly after terrorist bombings in Madrid that killed 191 people, earlier this year by a group using the same name that threatened to bomb railway targets unless the government paid them millions of euros.