A sixth person has died in hospital following a knife attack on Jersey, police said.
The woman was left critically ill following reports of a multiple stabbing at a flat at Victoria Crescent, Upper Midvale Road, St Helier, just after 3pm.
The victim has since died at Jersey Hospital, a States of Jersey Police spokeswoman said tonight.
The victims are two women, two men and two young children who are believed to be Jersey residents but whose identities have not been disclosed.
A 30-year-old man is under arrest at Jersey Hospital where he is recovering from surgery, police added.
Eyewitness Andre Thorpe said he arrived in Victoria Crescent in St Helier at about 3pm this afternoon.
He said: "For Jersey it was a major incident. Two ambulances turned up first because it was within a quarter of mile of the ambulance station.
"Then four or five police vehicles came, all blues and twos.
"They were trying to access a private house in the crescent. It was an old Victorian terrace, a lot of them are split into flats.
"I saw police come running out with a child, it was a small child, I just saw the legs. They went off in an ambulance.
"When the paramedic came back her shirt was covered in blood.
"We have quick response ambulance cars and two pulled up grabbed a kit and ran inside."
Mr Thorpe also said he saw police taking a dog round various driveways.
He added that the area where the incident happened was quite secluded and not on a main road.
He said: "You have to drive up a dead end to get in, you would not happen to be passing. I was in the area collecting a friend's car.
"I was first aware something was happening when I saw people looking out for the ambulances as I walked up the road."
Mr Thorpe said he believed all the victims were members of the same family.
A neighbour said she believed the family were Polish.
A major incident room has been set up at police headquarters in St Helier.
A number of witnesses have come forward and police are working to identify the victims.
Forensic officers were at the scene, with uniformed officers and detectives.
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.
"We are appealing to anyone who may have any information to contact us.
"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."
Jersey Hospital said that its emergency department remains open following the incident.
Police urged anyone with information to contact them on 01534 612612.
The hospital's emergency department was closed for more than two hours following the attack, believed to be because of the number of victims admitted.
A spokeswoman said it was closed from 3.10pm and reopened at 5.50pm as the hospital declared a "major incident".
"The emergency department reopened at 5.50pm, and the emergency department and the rest of the hospital are now open as normal," she said.
At a news conference, Mr Gull was unable to indicate how young the children were or confirm suggestions locally that the victims were Polish.
He said Jersey was an "incredibly safe" island, adding that it had been a "pretty traumatic" incident for emergency teams to deal with.
Mr Gull, who led the Ipswich serial murders inquiry in 2006, said: "We took a report from a member of the public to say that he was aware of an incident in the street.
"It now appears that a number of victims have been attacked with a knife or knives." He added it has been "challenging and quite complex" for emergency teams.
Mr Gull went on: "Jersey is an incredibly safe island, probably one of the safest places in the western world and incidents of this nature are an extremely rare occurrence, particularly in Jersey and across the UK.
"This is complex. We are trying to piece together exactly what happened this afternoon."
On whether he was shaken by the deaths, Mr Gull told reporters: "It goes without saying that when you are dealing with multiple deaths, of men and women and, in particular young children, you would be inhuman not to be shaken yourself."
Staff were called in on their days off to help at the hospital following the attack, it emerged later.
Angela Body, director of operations at the general hospital, said: "I thank all our staff for coming in to assist today.
"The teamwork spanned across many service areas including the ambulance, medical, nursing and support staff who all helped with a serious and unusual situation.
"As ever, staff worked quickly and in a professional way to help those who were brought in for treatment and care.
"I would like to thank those members of the public for their patience during this time.
"Our thoughts are with the friends and families of those involved at this sad and difficult time."
Some of the attacks happened in the flat and some occurred in the street, police said.
It is believed that detectives are not looking for anyone else but police inquiries are continuing.
A force spokeswoman said: "This is a close knit community and we have had a fantastic response with information from locals that they believe would be useful."