France’s anti-terrorism prosecutor opened an investigation on Monday into “murder in connection with terrorism” over Sunday’s attack on humanitarian workers in Niger, in which eight people were killed including six French citizens.

The prosecutor’s office is delegating the investigation to the DGSI – France’s internal intelligence agency, equivalent to Britain’s MI5 – and SDAT, the French police’s national counter-terror unit.

French and Nigerien soldiers on Monday combed a wildlife reserve in Niger and the surrounding area for traces of the gunmen who killed the aid workers, a French military source told Reuters.