A rapid action response team battered down the door of the man’s fourth floor apartment in Renens, a suburb west of Lausanne, just as he stabbed the infant, Le Matin reported online.
The incident occurred on Saturday at around 7.20pm.
At the same time as the team entered the apartment “the father stabbed the baby and tried to commit suicide by slashing his neck”, police said in a statement.
The child was rushed to the hospital but died of his injuries, while the man, whose age and identity was not disclosed, was treated for minor injuries.
“Everything happened in a few seconds and at the same time,” cantonal police spokesman Jean-Christophe Sauterel told a press conference on Monday.
Several witnesses said that the time between the sound of the baby’s cries and the entry into the apartment by police “seemed to be an eternity, maybe 10 or 15 seconds but it was long”, Le Matin reported.
Police had attempted to negotiate with the man for several hours before taking the decision, in consultation with the local prosecutor, to enter the apartment “by surprise”, the force said in a statement.
Police said they made the decision because officers were convinced the life of the child was in danger.
The incident began when the accused travelled by train to meet his former partner and child at the Neuchâtel train station on Saturday morning, police said.
The woman agreed to let the man see the baby and they stayed in Neuchâtel for about an hour before the woman said he threatened her, forcing her to drive him to his apartment in Renens, police said.
Once arrived in Renens at around 3pm, the man took the baby to his fourth-floor apartment and locked the door, according to the police information.
Fearing for the life of her son, the woman phoned police who responded with special negotiators and the rapid response team.