A nationwide hunt for the prime suspect in the Clapham corrosive substance attack has entered its fifth day.
Abdul Shookor Ezedi has been on the run since a woman and her two young girls were hurt in south London on Wednesday.
The convicted sex offender – thought to be from Afghanistan – was last seen leaving Tower Hill underground station about two hours after the attack.
Police say anyone found to have harboured or helped Ezedi, 35, will be arrested.
A £20,000 reward for information leading to his capture has been offered by the Metropolitan Police.
Police forces across the country have been put on high alert and UK Border Force and the National Crime Agency are assisting the search.
A 31-year-old woman, who is known to Ezedi, remains in hospital in a critical but stable condition with injuries “likely to be life-changing”.
Her daughters – aged three and eight – suffered injuries which are not “as serious as first thought”.
Nick Aldworth – a former senior counter-terrorism police officer – told BBC Radio 4’s Today programme police forces offer rewards for information “when there is a sense someone is hiding within the community”, possibly with help from others.
He said it was also “not unlikely or improbable” the suspect has taken his own life and “there is a body to be found”.
Mr Aldworth added it would be difficult for someone to move around London publicly for several days without being picked up on CCTV.
Before this week, 35-year-old Ezedi was living in the Newcastle area. He is believed to have travelled to the UK on a lorry from Afghanistan in 2016.
Police say he appears to have sustained a serious eye injury, which could make him more recognisable.
Former Northumbria Police chief constable Sue Sim, who led the week-long manhunt for killer Raoul Moat in 2010, said the police rely on the public to be their “eyes and ears” during large-scale searches.
Asked if police would be monitoring trains heading for Newcastle and accident and emergency rooms, she told BBC Radio Tees: “Yes. What we’ve got here is an offence that has happened in the Metropolitan Police area, and the fact he lived in the Northumbria Police area.
“And of course, the trains and the British Transport Police are all working together to try and track him down.
“But what you’ve got to remember with this man is he smuggled himself into the country in the back of a lorry. He is a very resourceful man so he could still be lying low.
“Every police force in the country will have been briefed on the fact he is wanted.”
Asked where Ezedi could be now, the former senior police officer said: “In a city it is very, very easy to become lost in a crowd.
“People are so busy with their lives, they keep their heads down, they don’t tend to look around themselves. It is much easier to be found in the countryside.
“But the city is difficult. He would probably want to go somewhere he knows or where he knows someone to get assistance.”
She said it was feasible he may have left the country illegally but that it is unlikely he would have been able to do so via legitimate routes.
Ezedi travelled to London from Newcastle in the early hours of Wednesday and the attack happened outside a hotel in Lessar Avenue at 19:25 GMT.
Police have previously confirmed they found a “very strong concentrated corrosive substance” after searching his Newcastle flat.
Ezedi was picked up on CCTV at a Tesco shop in Caledonian Road, Islington, north London, at 20:38.
At 21:00, he was seen on camera entering King’s Cross underground station and boarding a southbound Victoria line train.
He arrived at Victoria Station in central London at 21:10 and changed trains to travel eastbound on the District line at 21:16, police said.
Ezedi was seen leaving Tower Hill underground station a few minutes later.
The BBC has confirmed Ezedi was convicted of a sexual offence in 2018.
The Crown Prosecution Service (CPS) said he was sentenced on 9 January 2018 after pleading guilty to one charge of sexual assault and one of exposure.
He was given a suspended sentence at Newcastle Crown Court and put on the sex offenders’ register for 10 years.
Ezedi was later granted asylum after two failed attempts. He was allowed to stay in the country after a priest confirmed he had converted to Christianity.