Three Kurdish women, including one of the founders of a militant group battling Turkish troops since 1984, were "executed" at a Kurdish centre in Paris.
Turkey's Anadolu news agency identified one of the victims as Sakine Cansiz, a founding member of the Kurdistan Workers Party, or PKK.
RTL radio reported that all three were shot in the head, but police would not immediately confirm the report.
French Interior Minister Manuel Valls, who visited the pro-Kurdish centre in Paris where their bodies were found, said the deaths were "without doubt an execution".
He called it a "totally intolerable act".
Hundreds of people, including Kurds, gathered on the street where the centre is located.
Police and firemen discovered the bodies at about 1.30am at the Kurdistan Information Centre.
The official spoke on condition of anonymity because he was not authorised to give his name in keeping with policy.
In Turkey, Huseyin Celik, the deputy chairman of Turkey's ruling party, said the attack appeared to be the result of "an internal feud" within the PKK, but did not provide any evidence to back the claim.
Turkey has resumed talks with the PKK with the goal of convincing the group to disarm.
Mr Celik suggested that the murders were an attempt to derail those talks.
The conflict between PKK and Turkish troops has claimed tens of thousands of lives since the rebels - who are seeking self-rule for Kurds in southeast Turkey - took up arms in 1984.
Turkey's Western allies also label the group a terrorist organisation.
The Kurdish minority comprises more than 20% of Turkey's 75 million people.