An hour before Chérif and Said Kouachi gunned down 12 people at the offices of satirical newspaper Charlie Hebdo, they texted Amedy Coulibaly, who went on to kill a police officer and four others at a kosher supermarket.
It was the first evidence that the brothers had coordinated their attack, French media reported on Tuesday.
According to Le Monde, the text message was found on one of 13 phones used by Coulibaly. The newspaper did not report exactly what it said.
Authorities also believe Chérif Kouachi and Coulibaly met the night before the Charlie Hebdo attack, Le Monde reported.
French television station BFM had previously aired comments by Coulibaly that the attack on a kosher grocery store was "synchronized" with that at Charlie Hebdo. Following his death and that of the Kouachis, however, authorities were left to investigate their communications and preparations for the attacks.