Police were called to a flat at Victoria Crescent, Upper Midvale Road, at 1500 hours after reports of a multiple stabbing incident.
A 30-year-old man is in police custody at Jersey General Hospital where he is also recovering from surgery.
Two men and two women died as well as two young children. Their identities and ages are unconfirmed but police say they were Jersey residents. It is believed they are all from the same family.
The hospital’s emergency department was temporarily closed to all non-urgent cases as staff treated the victims.
A number of witnesses have come forward and are currently being interviewed by police as officers make efforts to identify the victims.
States of Jersey Police head of crime services Stewart Gull, who is leading the inquiry, said: “Clearly this complex investigation is in its very early stages as we try to establish exactly what happened.
The scene of the killings has been cordoned off “We are mindful, too, of the impact of any such serious and tragic incident on the local community, and we have police officers in the area to support the investigation and local people.”
Mr Gull, who led the inquiry into the killings of five Ipswich prostitutes in 2006, admitted he was shaken by the Jersey incident and that it had been “pretty traumatic” for emergency services staff called to the scene.
“Dealing with multiple deaths of men, women and particularly young children, it has shaken the force but the officers who initially responded were tremendously professional,” he said.
“Jersey is an incredibly safe island, probably one of the safest places in the Western world. Incidents of this nature are an extremely rare occurrence.”
This has got shades or Norway about it. Jersey is a place that most people would have thought was immune to such atrocities. It seems however that evil knows no borders.