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.
Mr Khalife clung to the underside of a delivery lorry to make his escape from HMP Wandsworth on Wednesday morning.