A question of identity

The murder of a very old woman in a housing project rocks the town of Lafferton. The murderer left a distinctive "sign" on the body. A couple of weeks later a similar murder occurs, and then another. Investigations reveal that the "sign" was the calling card of a suspect in several murders in the northwest of the country. Tried but acquitted due to insufficient evidence, this person has vanished. Now Chief Superintendent Simon Serrailler is obliged to delve deeper.