Two students and a man in his 50s were stabbed to death and a van tried to mow down three other people in Nottingham, central England, on Tuesday, in incidents the authorities believe are linked.
Nottingham's centre was cordoned off, with a heavy police presence, including armed officers, following the events that left residents shaken.
Police said a 31-year-old man has been arrested on suspicion of murder but they were not looking for anyone else.
Counter-terrorism police have been drafted in to help with the investigation but there has been no formal declaration that it was a terror incident.
Nottingham University — one of two universities in the city — confirmed that two of its students were among the dead.
"We are shocked and devastated by the news and our thoughts are with those affected, their families and friends," a statement read.
Police said the victims were aged 19.
Prime Minister Rishi Sunak thanked emergency services who had dealt with the "shocking incident" in the city, which is home to more than 320,000 people.
"My thoughts are with those injured, and the family and loved ones of those who have lost their lives," he said.
Screams
Police were called just after 4:00am (0300 GMT) after the two students were found in Ilkeston Road, which runs west out of the city centre between Nottingham University and Nottingham Trent University.
Residents reported hearing screams and seeing a young man and woman being stabbed, before the attacker — said by one witness to be dressed in black with a hood and rucksack -- walked off calmly.
The man in his 50s was then found stabbed on Magdala Road, about two miles (3.2 kilometres) away, police said.
The three people hit by the van, in Milton Street, in the city centre, were being treated in hospital, the force added.
One was in critical condition while the others had minor injuries, it added.
A white Vauxhall van with a shattered windscreen was cordoned off a mile away outside a convenience store, an AFP photographer said.
A black rucksack was seen on the road under the vehicle's open passenger-side door.
Sirens
Witness Lynn Haggitt said she saw a van hit two people at around 5:30am near the city's Theatre Royal after the vehicle pulled up beside her on her way to work.
"He looked in his mirror, and saw a police car behind him, he then quickened up. There were two people... He went straight into these two people," she told the BBC.
Glen Gretton, a 46-year-old delivery driver, said he was woken up at around 5:00 am by the sound of police cars passing his home.
"They just kept coming so I knew something quite major... was happening somewhere around the city centre," he said.
Nottinghamshire Police Chief Constable Kate Meynell described the incidents as a "tragic series of events" and a "sad day for the city", but stressed the motive was so far unclear.
The van is believed to have been stolen from the older victim, she told a news conference, adding that the man arrested was tasered.
"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," she added.
The city's tram network was suspended while the investigation took place, with several streets still blocked to pedestrians and traffic.
The city's three members of parliament — Nadia Whittome, Lilian Greenwood and Alex Norris -- said they were "shaken" by the events.
"Our city has been devastated by the deaths of three people this morning. Nottingham is a beautiful city, home to brilliant people from all backgrounds.
"We are shaken by today's events but will meet them collectively as a community and heal together," they said.
