Nottinghamshire Police say they are not looking for any other suspects and are working with counter-terrorism teams to establish the facts.
Police are keeping an open mind over the motive for a knife and van attack in Nottingham that left three people dead.
A 31-year-old man has been arrested on suspicion of murder after a deadly rampage in the city on Tuesday and detectives are not looking for anyone else in connection with the violence.
Nottinghamshire Police Chief Constable Kate Meynell said the force is working with counter-terrorism investigators to establish what happened.
Two people were stabbed to death in a student area of Nottingham at about 4am while the body of another victim was found in Magdala Road.
Three people were hurt, one critically, when a suspect driving a van tried to run them over near the Theatre Royal.
Major roads in the city centre are set to remain closed as the police investigation continues.
Ms Meynell said: “This is a tragic series of events which has led to the lives of three innocent people being taken and left another member of the public in a critical condition in hospital.
“My thoughts are with all the families affected by this shocking incident and we will be working extremely hard to understand exactly what has happened.
“We are at the early stages of the investigation and need to determine the motives behind these attacks, and will keep the public updated as soon as we are able to say more.
“We are keeping an open mind as we investigate the circumstances surrounding these incidents and are working alongside Counter Terrorism Policing to establish the facts – as we would normally do in these types of circumstances.
“We do have a man in custody who has been arrested on suspicion of murder. Currently, we do not believe there is anyone else involved in this incident.
“It is safe to go into the city centre but there are a number of streets that will remain closed including Ilkeston Road, Magdala Street, Milton Street and Maple Street. This is so officers can gather evidence in order to understand what has happened.”