An on-the-run killer who was sprung from custody by two armed men during a hospital visit was taken there by taxi, the BBC understands.
Convicted murderer Shaun Walmsley, 28, was getting into the cab with prison officers at the time of the escape.
The getaway car had lain in wait in a road near Liverpool's Aintree University Hospital, police said.
Mayor of Liverpool Joe Anderson called for an "immediate inquiry" into the escape.
Walmsley, described as "highly dangerous", is one of four men serving life sentences for the "vicious and savage" stabbing of Anthony Duffy in 2014.
Mr Anderson said he was "angry and incredibly shocked" by the "serious security breach" and is waiting for a full briefing into what happened.
Deputy General Secretary of the Prison Officers' Association (POA), Andy Darken, said it is "fairly common" for non-Category A prisoners to be moved in taxis after a risk assessment is done to ensure this is safe.
"There's not a lot of difference if they are held up by armed gunmen in a secure van or a taxi", he said.
Two men, armed with a gun and a knife and with their faces covered, threatened the guards and demanded they release Walmsley on Tuesday.
Mark Fairhurst, who sits on the POA national executive and is based at HMP Liverpool where Walmsley was being held, said the incident "put our officers' lives - and that of the taxi driver - at risk."
He questioned why Walmsley, "a man convicted of murder facing a 30-year sentence and who had gangland connections" was not treated as a Category A prisoner.
He said the prison recognised Walmsley was "more dangerous than others" so "put a third prison officer into the escort" but they "had no deterrent capability - just extendable batons".
He said one of the officers had a gun put to his head and was "very shaken", while another had a knife held to his throat.
All three were threatened with CS gas.
He said the officers did the right thing in releasing the handcuffs and then calling police.
A Ministry of Justice spokeswoman declined to comment on the specifics of the case.
"We are working closely with the police and are urgently investigating the matter," she said.
Officers believe a gold-coloured Volvo with registration number MW02 XHE had been parked on Adlam Crescent, Fazakerley, from around 12:00 GMT before travelling to and parking at Aintree University Hospital from about 13:30.
The car was found in Adlam Crescent at 20:00 on Tuesday and is being examined by officers.