An overnight search of Richmond Park in south-west London was in connection with Daniel Khalife, the terror suspect not seen for near 48 hours since escaping prison, police have confirmed.
The 21-year-old is accused of trying to spy for an enemy state, understood to be Iran, and plotting a fake bomb hoax.
Two police helicopters were seen over the park – one of the biggest in the capital – during the night.
It is located 2.8 miles (4.6km) from Wandsworth prison.
The aircraft were seen flying over the park around 23:30 BST on Thursday.
The Met Police commander leading the search said the former soldier was “very resourceful”.
Mr Khalife clung to the underside of a delivery lorry to make his escape from HMP Wandsworth on Wednesday morning.
Detectives believe he was still wearing his prison-issue cook’s uniform when he slipped out of the category B jail.
There have been no confirmed sightings of Mr Khalife since he was declared missing.
Cdr Dominic Murphy, head of the Met’s Counter Terrorism Command, said it was a “little unusual and perhaps a testament to [Mr Khalife’s] ingenuity” that he had not been spotted, despite 50 calls from the public offering “valuable lines of inquiry”.
As a trained soldier “he has skills that perhaps some sections of the public don’t have”, he added.
Cdr Murphy said it could not be ruled out that Mr Khalife had had help escaping and had access to money.
Some 150 Met Counter Terrorism Command officers have been deployed in the search since he was declared missing at 08:15 BST on Wednesday.
The lorry carrying Mr Khalife left HMP Wandsworth at 07:32 BST and headed west – but the alarm was not raised by prison staff for a further 20 minutes.
Another 25 minutes passed before the police were called in.
Mr Khalife is thought to have links with Kingston upon Thames, south-west London, and the north-west of England. The search is focused on London.
But Cdr Murphy said it was “absolutely possible” he might have already left the country, and security at ports and airports has been tightened.
Disruption continued at the Port of Dover on Thursday. Junctions eight and nine of the M20 were temporarily shut due to the enhanced checks.
Meanwhile, a man took to X, formerly known as Twitter, to recount how a member of the public mistook him for Mr Khalife and alerted the police.
Pictures on social media show the man being spoken to by police at Banbury train station in Oxfordshire on Thursday but Scotland Yard said it was not Khalife.
The man said he proved he was not the escapee after being questioned by police for 20 minutes and a fingerprint scan.
“Honestly crazy how much this has blown up,” he tweeted. “Mistakes happen.”