French investigators say a letter from an Algerian militant group is an implicit claim of responsibility for a deadly Dec. 3 subway bombing in Paris, Le Monde newspaper reported Wednesday.

The report quoted the letter as saying: “We do what we say (we are going to do). The events of recent days prove that.”

The newspaper said investigators now deem the letter to be almost certainly authentic. The government said Tuesday that it had not yet authenticated the document, and Muslim fundamentalists in Algeria had cast doubt on its provenance.

Algerian Islamic militants accuse France of supporting the secular government in its former North African colony and have threatened to step up attacks on French soil.

The letter was reportedly sent last week, but its existence was made public only Tuesday. French media said it was addressed to President Jacques Chirac and threatened to “destroy your country” unless aid to Algeria was cut off and imprisoned militants freed from Algerian jails.