The events in Salisbury on 4 March were unprecedented, triggering one of the largest and most complex investigations by British counter terrorism police.

As widely reported, Sergei Skripal, a 66-year-old British national, and his 33-year-old daughter Yulia, a Russian citizen, were found at 4.15pm unconscious on a bench near The Maltings shopping centre in Salisbury, having been exposed to a military-graded nerve agent.

Both spent several weeks critically ill in hospital. Thankfully they are both now making a good recovery after being discharged from hospital – Sergei was discharged from hospital on 18 May and Yulia was previously discharged on 9 April.

Detective Sergeant Nick Bailey, a Wiltshire police officer who was amongst the first to respond to the incident, also fell seriously ill after being exposed to the nerve agent and was admitted to hospital on 6 March. Since being discharged from hospital on 22 March, Nick has continued to make good progress but remains off work.

Over the last three months, 250 detectives, drawn from across the Counter Terrorism (CT) Policing Network, have been involved in the painstaking investigation into the attempted murder of the Skripals.

Today, around 100 counter terrorism officers remain in Salisbury. They continue to sift through and assess all the available evidence and are following every possible lead to identify those responsible.

To date 176 searches have been undertaken in Salisbury, while more than 900 witness statements have been taken.

Meanwhile staff from the national CT network, along with colleagues seconded from five forces including specialists in image identification, have now viewed over 4,000 hours of CCTV footage.

Officers have sought to identify all pedestrians, cyclists and vehicles that travelled through the relevant area at the times in question. In total 14,000 vehicles and 2,500 pedestrians have been assessed and graded according to their significance.

Over 2,300 exhibits have been recovered as part of the investigation, of which 851 are stored at the Defence Science and Technology Laboratory because of contamination issues. Detectives have so far carried out 190 examinations at the Porton Down laboratory.

House-to-house inquiries have been completed at 379 addresses, while officers have dropped leaflets appealing for information at hundreds more homes.

Detectives continue to focus their enquiries around the Skripals' home address where it believed that they first came into contact with the nerve agent. Specialists have identified the highest concentration of the nerve agent as being on the front door of the address. Traces of the nerve agent were found at some of the other scenes detectives have been working at over the past few weeks, but at lower concentrations than found at the home address.

Deputy Assistant Commissioner Dean Haydon, the Senior National Coordinator for Counter Terrorism Policing, said:

"Clearly this is a very unusual case – both in its scale and complexity. We have said from the start that this investigation was going to take some time as we rigorously follow the evidence.

"We continue to deal with a number of unique and complex issues in what is an extremely challenging investigation. Some of our leads have come from members of the public and I would like to thank the people of Salisbury for their help, support and patience. The city continues to recover from the incident.

"With such a sensitive and complex investigation, I am sure the public will appreciate that there are still a number of lines of inquiry being progressed that we cannot discuss at this stage.

"What I am able to say is that the team working on the investigation remain completely committed to finding out what happened and continue to follow every possible lead.

"Our enquiries are focused around the people and vehicles that were in the vicinity of the Skripal's address and leading up to where they fell ill in The Maltings. We would ask anyone who may have information, or who may have seen or heard something – however small - to contact police on 101."

Throughout, CT Policing has worked closely with Wiltshire Police, colleagues seconded from other forces and a range of partner agencies including Public Health England, Defra and DSTL. In total 1,230 officers from an additional 40 forces across the country have been part of the police response with large numbers required at the many cordons in the city.

In the interests of Sergei and Yulia's safety, we will not be discussing any protective or security arrangements that are in place.

Timeline of events (all times approximate):

14.40hrs on Saturday 3 March: Yulia arrives at Heathrow Airport on a flight from Russia.

09.15hrs on Sunday, 4 March: Sergei's car is seen in the area of London Road, Churchill Way North and Wilton Road.

13.30hrs: Sergei's car is seen being driven down Devizes Road, towards the town centre.

13:40hrs: Sergei and Yulia arrive in Sainsbury's upper level car park at the Maltings. At some time after this, they go to the Bishops Mill Pub in the town centre.

14.20hrs: They dine at Zizzi Restaurant.

15:35hrs: They leave Zizzi Restaurant.

16.15hrs: Emergency services receive a report from a member of the public and police arrive at the scene within minutes, where they find Sergei and Yulia extremely ill on a park bench near the restaurant.