Police began searching an area of the River Thames in west London for chemical attack suspect Abdul Ezedi on Saturday morning.
A police boat was seen circling between Vauxhall and Chelsea bridges, with CCTV footage indicating that this area was near where the 35-year-old headed following the incident.
Ezedi is wanted after a woman and her two children were injured in the attack in Clapham on January 31.
He was last caught on CCTV heading on to Chelsea Bridge for a second time, two minutes after he had first walked across it. Cameras did not capture him leaving.
The force said on Friday that boats from the Marine Policing Unit would be used as part of the search and it would take place at low tide - although the nature of the Thames makes it difficult for authorities to find bodies.
Commander Jon Savell said: "We have looked at all of the available cameras and angles, and with the assistance of Transport for London and CCTV from buses that were travelling over the bridge at the relevant time, and there is no sighting of him coming off the bridge.”
He told a Scotland Yard press conference on Friday that the "main working hypothesis" was that he had gone into the water.
"[It may] take some time... for a person to surface and sadly, they may never actually be found,” he added.
Detective Superintendent Rick Sewart said that "most probable outcome” is that Ezedi would have died if he entered the water.
The victims, a 31-year-old mother and her two daughters, were hospitalised after being attacked with the alkaline substance. The woman was known to Newcastle resident Ezedi and remains in hospital but the children have been discharged.
Afghani-born Ezedi arrived in the UK in 2016 but had several asylum claims turned down - before he was able to reportedly gain asylum after claiming religious persecution. He previously committed sexual offences in 2018.
He is wanted as the prime suspect on suspicion of attempted murder.