The question “what if?” plagues Smera Chohan and her husband, Sajjad Butt. Last July, Chohan had just posed for a photograph with their eight-year-old daughter, Nuria, at her school picnic in Wimbledon when a Land Rover crashed through a fence and ploughed into them. Nuria Sajjad and her friend Selena Lau were ultimately killed by the collision while Chohan was left with 10 broken bones and the overwhelming grief of losing her only child. Slow progress in the police investigation means that 10 months after the girls’ deaths, their families still have no answers as to what happened. Speaking at their home in south-west London, Nuria’s father said: “In the absence of concrete answers, in the absence of steps moving forward, we’re left to ponder this ‘what if?’ – and the ‘what ifs’, they crush you.” This week it was reported that the Metropolitan police force has been unable to progress its investigation further because of a chronic shortage of specialist forensic investigators. The Guardian understands that there were about 40 specialist forensic collision investigators in the Met in 2010 – but now the number is down to 11, with another 17 in training. The apparent stagnation of the Wimbledon crash investigation has only made it harder for the families affected. A woman was arrested last year on suspicion of causing death by dangerous driving and released under investigation. No decision has been made on whether any charges should be brought. The alternative scenarios that day haunt Nuria’s parents. They have a photo of Chohan and Nuria standing on the grass outside the Study preparatory school and another in front of the building, both taken within the same minute. “I question myself that had I just stayed in the grassy patch, the outcome would have been different, right? There are so many things that are ‘if only’. If only it was raining that day …” Chohan said, before Butt finished the sentence: “We wouldn’t have been outside, we would have been in the hall.” The families’ lawyer, Trevor Sterling, said delays to investigations came at a high psychological price: “You need to have the facts so that you can rationally say to yourself, ‘Of course there was nothing more I could do’ … PTSD and the psychological impact is only made worse in the absence of the evidence, the facts, the answers.” In a letter to Sterling, the Met commissioner, Mark Rowley, said the force was facing a shortage of specialist forensic collision investigators. Rowley said the case had not progressed further because the police were waiting for specialists to finish a report on the incident. Sterling believes the shortage in experts needs to be tackled urgently. “Delay itself causes prejudice and injustice. This isn’t a one-off, this is a wider problem that needs to be fixed,” he said. Mark Crouch, who chairs the Institute of Traffic Accident Investigators, worked in the Met before moving into private forensic collision investigation. He said that in around 2010 there were about 40 specialist investigators but the number “was slowly dwindling” by the time he left in 2016 and the crisis had “been coming for a long time”. Crouch said the shortage of specialist investigators had come at the same time as the workload for each case had increased because of advances in car technology and CCTV as well as an increase in regulation. In addition to causing delays in investigating fatal cases, Crouch said that in many serious injury cases, “specialist units aren’t going out and collecting the evidence”, making it harder for victims to get justice. “There are people that can never walk again or in a vegetative state that don’t get any help at all, because evidence wasn’t gathered,” he said. Nick Simmons, the chief executive of RoadPeace, which campaigns on behalf of collision victims, said its helpline was increasingly hearing from crash victims frustrated by the lack of appropriate forensic investigations. “It is clear there is a real shortage of resources across the UK and that an already devastating experience is somehow made worse by victims not being able to seek justice,” he said. Nuria’s father believes it is “grossly unacceptable” that there appears to have been no resolution over several years to the shortage of forensic specialists, and feels that “nobody should have to suffer this”. But ultimately, Butt is less interested in the reasons for the delay than in the investigation concluding. “I just want justice for our girl,” he said. Their home was still full of the accoutrements of childhood: a trampoline, climbing frame and swing sat in the garden unused, and an edition of the school’s newsletter sat on the coffee table. Photographs of Nuria were everywhere and much of the house was as she left it – right down to the birthday presents she was saving, kept safely in a cupboard. Chohan was chair of the parent-teacher association at the school, and the couple say the support from fellow pupils’ families has been “truly amazing”. Nuria’s friends still visit, but her parents are struggling to adjust to a world without their daughter in it. “I still go to the mums’ coffee morning,” Chohan said, her face suddenly crumpling as she added: “Although I’m not a mum any more.” Butt was quick to counter her, saying softly: “You’ll always be a mum.” After the interview, Chohan headed to the gym for physiotherapy. After four operations and hard work she has gone from using a wheelchair to having most of her mobility restored. Her next hurdles are psychological. She has not yet been able to face going back to the school, but is steeling herself to try in July. “I plan to go back on the anniversary to claim my happy space back. That’s part of my own healing journey. That’s where Nuria was the happiest and that was where she smiled for the last time in her life.” DCS Clair Kelland, who is in charge of policing for south-west London, said: “This was a tragic incident and we understand that the families want and need answers as to what happened.” She said specialist detectives had established the circumstances, and advice had been obtained from the Crown Prosecution Service. The force was now providing specialist support and updating the families on the investigation where possible, including why “one specific issue” had been delayed due to a lack of specialist investigators. She added: “We recognise that the time taken has caused further distress but it is only right and fair to all involved that we carry out a thorough and extensive investigation.”