The DNA of the man, named as Eric D by Le Parisien newspaper, did not correspond to any found at the scene of the crime.
But a number of guns were found at his house and a friend of his was arrested while trying to flee after police tried to carry out a search of his home.
Public prosecutor Eric Maillaud admitted on Wednesday that he was unlikely to be charged for the murders, despite his strong resemblance to an identikit picture of a man spotted by witnesses, but said he was now under suspicion of illegal arms trading.
His detention was extended on 48 hours when the first legal limit was reached on Thursday and his friend also remained in custody.
Eric D had a firearms permit thanks to a job in security, according to Le Parisien.