A violent armed robber dubbed the "Skull Cracker" is believed to have travelled to London by train after absconding from an open prison.
Michael Wheatley, 55, was released on a temporary licence from HMP Standford Hill open prison on the Isle of Sheppey, Kent, on Saturday, and is thought to have boarded the 9.20am high speed train from Sittingbourne, which was heading to Stratford International station, Kent Police said.
A 999 call was made to police when Wheatley failed to return to the prison at 6pm and a specialist team of officers, headed by detective chief inspector Ann Lisseman, have pieced together a timeline of his movements since his disappearance.
Inquiries have led the officers to work with neighbouring police forces, including the Metropolitan Police, Hertfordshire Constabulary and British Transport Police.
It is believed that Wheatley travelled by train from Sittingbourne to the London area, police said.
Officers are keen to hear from anyone who travelled on that train who thinks they may have seen Wheatley.
Officers also responded to a confirmed sighting of Wheatley at 7.55pm on Monday at a property in the Strawberry Hill area of Twickenham.
This led police to search several other properties in the Twickenham area, Kent Police said
Ms Lisseman said: "Our inquiries are on-going and we have several leads to follow.
"We are aware that Michael Wheatley has past associations in Ireland, Cheshire and North Wales and we are working with numerous police forces as part of our efforts to find him.
"People who know the whereabouts of Michael Wheatley are asked not to approach him but call the police via 999."
Wheatley, who was given 13 life sentences in 2002 for a series of brutal raids on banks, is one of more than 1,200 open prison inmates serving an indeterminate sentence.
His disappearance has prompted ministers to launch a major review of the case, including a broader assessment of the release on temporary licence (ROTL) process.
Most recent Ministry of Justice figures show there were 1,242 indeterminate sentenced prisoners - that is, those serving life and imprisonment for public protection sentences (IPP) - as at December 31 last year. This includes 643 lifers and 599 IPP inmates in open prisons.
Scotland Yard said they were called to an address in Twickenham, south west London, last night following a sighting of Wheatley, who raided 13 building societies and banks over 10 months in 2001 and 2002 while on parole from a 27-year sentence for other robberies, but he was not found.
Wheatley, originally from Limehouse in east London, earned his nickname after pistol-whipping victims, including a 73-year-old woman, during the raids.
He admitted 13 charges of robbery and 13 of possessing an imitation firearm - a blank firing semi-automatic pistol - in October 2002.
The robberies between June 2001 and April the following year were mainly on small branches in areas Wheatley knew, ranging from Southampton in Hampshire to Royston in Hertfordshire.
The first was just three weeks after he was paroled from his first prison term. As the robberies continued, so did the violence he used towards staff and customers.
The Old Bailey heard at the time that he would often grab a female customer, putting the pistol to their head, leaving many mentally anguished.
His raids netted him more than £45,000.
He was given a five-year sentence on each of the firearms offences to run concurrently with the life sentences on each of the robbery charges. He was ordered to serve a minimum of eight years before being eligible for consideration for parole.
Prisons Minister Jeremy Wright said temporary licence can be an important tool to help offenders reintegrate into communities but that "it should not be an automatic right".
Ministers have said there will be a toughening-up of the licence scheme so that prisoners are subjected to stricter risk assessments and tagged.
Mr Wright said: "In future, when prisoners are let out on temporary licence, they will be tagged, more strictly risk-assessed and tested in the community under strict conditions before being released.
"Temporary release can be an important tool in helping offenders reintegrate but it should not be an automatic right.
"There will be a full review of this case which will look at the ROTL process."
Juliet Lyon, director of the Prison Reform Trust, said: "Of course there should be a review into any breach of safety and security but, to put things in perspective, Government figures show the main lessons to learn from open prisons are that the Prison Service has achieved a year-on-year reduction in absconds and that release on ROTL has succeeded in significantly reducing the risk of re-offending."
Conservative backbencher Philip Davies said that whoever had allowed Wheatley out of prison was "a berk" and questioned why he was in an open prison in the first place.
The MP for Shipley in West Yorkshire said: "It is completely ludicrous that a serving life sentence prisoner is even in an open prison where they can simply walk out.
"As far as I am concerned, whoever allowed him to be in an open prison should be sacked. It is a complete disgrace.
"The top priority for the Prison Service should be the protection of the public. (Justice Secretary) Chris Grayling needs to put in charge of the Prison Service someone who will see protection of the public as a top priority."