The 26-year-old homeless man, who claimed to hear voices, had passed army medical tests without any problem, said a senior source at the Marine Infantry Tank Regiment, a light cavalry regiment of the French army.
The source at the regiment, one of the army's most decorated units, said the man had "posed no problems during his stint" in Afghanistan, where he obtained the grade of corporal.
When he returned, the man refused an army offer to extend his contract, saying he wanted a painting job, the source said, without specifying when the man had served.
He left the regiment towards the end of October, and only left active military service as recently as November 5th, said the same source.
The young homeless man on Thursday attacked the elderly man in a tiny hamlet, smashing his head with a metal object, and then extracted his heart and tongue to cook and eat.
Some remains of cooked meat were found on a plate with some beans.
Post-mortem tests have shown that a part of the heart and the tongue were ripped out, but it is not yet clear if the meat found was of human origin.
The suspect assaulted another man, breaking his shoulder, before being arrested.
Psychiatric tests after the arrest showed the man suffered from delusions.