There is a gathering sense here tonight that this investigation is finally gaining momentum. This was a crime, which of course shocked, but quite swiftly seemed to baffle police, as they searched for a motive and then it seemed to frustrate them, as they searched for evidence.
As much as French investigators are trying to play down events here, they say the man being questioned here is being questioned as a witness, not as a suspect but it is really hard to disguise the idea that a breakthrough might be imminent.
The man we know was a former policeman. It has been reported by French media that weapons were found at his home. French police have four days to question him before charging but tomorrow the prosecutor is talking about holding a press conference. That suggests there is something significant to say.