Three passengers in a minibus carrying physically and mentally handicapped adults were killed tonight in a collision with a lorry.
Police said eight other passengers and the driver of the minibus were injured in the accident outside Toomebridge, Co Antrim, on the main road between Belfast and Derry.
At least two of the injured taken to the Antrim Area Hospital were believed to be in serious conditions.
The driver of the lorry was also taken to hospital, but police said the 40-year-old from Derry was later arrested for causing death by dangerous driving and was assisting them with their inquiries at a local police station.
The minibus is understood to have been carrying people from a local nursing home.
The accident happened outside a petrol station and restaurant and an eyewitness Mr Peter Doran, who was having coffee there, said: "It was an absolutely terrible thing to see.
"I saw the minibus sliding on its side along the road, it hit a wall and came to a sudden stop."
He ran from the restaurant shouting for someone to call an ambulance.
The ambulance service praised those who helped the injured before the emergency services arrived.
A spokesman said they did "an absolutely marvellous job".
The Sinn Féin MP for Mid Ulster Mr Martin McGuinness tonight expressed sympathy for the victims and survivors of today's accident.
The Stormont Education Minister, whose constituency borders on the scene of the crash, said: "This is a terrible tragedy, especially for those involved who had learning difficulties.
"My thoughts go with the injured and those who have lost loved ones.
"We obviously wish those in hospital a speedy recovery," he added.
Mr Donovan McClelland said: "People are in absolute shock about this accident. They cannot believe it has happened.
"It is all the more shocking as it involved people with learning disabilities. That gives an added poignancy. There is a very special sense of grief.
"Details are sketchy about how the accident occurred and locals will be waiting to see what more information emerges about the circumstances of the accident."
The Northern Ireland Assembly's environment committee chairman, the Reverend William McCrea, who was travelling on the road where the accident occurred, described it as a "terrible tragedy".
The Democratic Unionist told reporters: "This is an appalling incident.
"My thoughts and prayers just go out to the families of those killed and everyone involved in this terrible tragedy.
"Everyone travelling on the road has been affected by this. It has struck very deep."