Searches have continued over the weekend along the River Tamar for 16 year old Jordan Cobb without success.
Police are asking the public not to carry out their own searching so that resources can be concentrated where they are most needed.
More top news
The man who carried out the Westminster attack is thought to be British born and was once investigated by MI5, the Prime Minister has said.
More than 50 primary school children from Somerset, who were caught up in the terror attack in Westminster, have returned home safely.
Police forces across the region have been paying tribute to an officer killed during the terror attack in Westminster.