French police have launched an investigation after death threats were delivered to the Finance Minister Bruno Le Maire and Minister of Public Accounts Gérald Darmanin.
Two bullets were enclosed in one of the letters which demanded the end of President Emmanuel Macron's push for pension reforms.
"You convince Macron that enough is enough. Tell him to call off the reforms otherwise...you're finished," read one of the letters.
Back on the tracks
Macron swept to power in 2017 on the promise of sweeping reforms for France.
Strikes against his retirement benefits proposals, which began on 5 December, have largely subsided, but not before causing weeks of severe disruptions to public transport around France, in particular to the Paris metro and bus network.
During the Christmas holidays, hundreds of thousands of travellers were affected due to limited or cancelled services.
SNCF, the national train operator, says it faces losses of up to one billion euros due to the strike action.
On 15 January, Prime Minister Edouard Philippe insisted the strikes would not succeed in derailing the government's plans.
"The transport strike against pension reform will go nowhere. The government is determined," said Philippe after emerging from a meeting with key ministers and advisors.