Merah's brother Abdelkader has already been charged with helping his sibling steal the powerful Yamaha scooter used in the series of shootings – including the murder of three young Jewish children – that shocked France.
A police source said the third person, whom police have not yet identified, was also possibly involved in buying scooter accessories for Merah and may have helped try to neutralise the anti-theft tracking device on the scooter.
Investigators believe Merah carried out his killings alone but may have received logistical help from his friends and acquantainces, and in particular from his elder brother Abdelkader.
Abdelkader Merah was charged on Sunday with complicity in the murders and remanded in custody. He was also indicted for involvement in preparing a terrorist attack as well as the theft of a scooter used in the shootings.
Mohammed Merah, who said he had links to Al-Qaeda, died Thursday in a hail of police bullets after a 32-hour stand-off with police at his apartment in Toulouse, southwest France.