British police said on Friday they had arrested a teenager under terrorism laws following the discovery of a “suspicious item” on a London underground train near the Canary Wharf financial district a day earlier.

Officers discharged a stun gun during the arrest of the 19-year-old in north London who was detained on suspicion of preparing terrorism acts, London’s Metropolitan Police (MPS) said in a statement.

“Officers are keeping an open mind regarding any possible motive. They are not looking for anyone else in relation to this investigation at this stage,” the statement added.

The suspect item, which is still being forensically examined, was found by staff on Thursday morning on a train at North Greenwich station in east London near Canary Wharf and close to the O2 music venue.