Officers were called to Ilkeston Road in the city at 04:00 BST where two people were found dead.
There was another incident where a van attempted to run over three people, who are being treated for their injuries.
A man was also found dead in Magdala Road in the city.
The sight of distinctively dressed officers apparently from an organisation called NILO fuelled speculation that something was seriously wrong in Nottingham.
But the officers in burgundy overalls and protective gear are not – as some people theorised on social media – a bomb disposal team or special forces. In fact they are National Inter-Agency Liaison Officers – highly-trained members of the emergency services who ensure that police, fire, ambulance, and other agencies work together during a crisis.
In order to become NILO qualified, fire officers must already have experience as incident commanders. According to the Fire Service College, NILO officers co-ordinate the response to “various terrorist and other malicious attacks, natural hazards, major accidents and other incidents”.
Developed by the London Fire Service in 2001 but now rolled out nationally, NILO teams are not intended to take charge of an incident, but to offer advice and experience to ensure the safety of emergency responders and the general public.
Counter-terror police involved in investigation from Nottinghamshire Police Chief Constable Kate Meynell.
“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,” she says.
“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.”