Detectives now believe it is "very likely" that a mother plunged 100ft to her death from the Humber Bridge with her son, who had a genetic condition with similar traits to autism.
CCTV footage recovered from a tower on the bridge shows two people climbing over the 4ft 6in railings and falling from a footpath shortly before 4pm last Wednesday, with about eight seconds separating the two falls. There were no signs of a struggle or physical violence.
The body of Ryan Davies, 12, who had fragile-X syndrome, was found a few miles downstream of the bridge on Sunday. His mother, Alison Davies, is missing.
Half an hour before the two people jumped, Ms Davies made a 999 call to Humberside Police from the vicinity of the bridge. But police in Greater Manchester, where the pair lived, did not find a suicide note in which Ms Davies indicated she was going to the bridge until 10.55pm last Wednesday, so Humberside did not dispatch officers when they received the call.
Detective Superintendent Colin Andrews, who is leading the investigation, said that Ms Davies' intention when she called 999 to ask police to tell her mother "not to worry" was unclear and was being investigated.
Ms Davies, 40, a single parent, suffered from depression and struggled to cope with her increasingly challenging son. They left her mother's home at Marple, near Stockport, last Tuesday and took a train to Hull - a city to which Ms Davies was emotionally attached, having once lived there.
Det Supt Andrews said: "Ryan was not forcibly dragged anywhere, there was no violence between them. When they arrived in Hull ... they appeared to be a normal mother and son. When you piece everything together... one has to be honest and come to the conclusion that it's very likely the people who fell from the bridge are Ryan and Alison."
Police are in contact with social services staff and are examining child protection issues. Ms Davies' sister, Lindsay Cook, has said that she wants to highlight "the ongoing plight of families who are coping with mental health and learning disabilities in order to spare other families from experiencing this pain."Reuse content