The first of five MI5 officers today began giving evidence at the trial of alleged Manchester bomb plotter Abid Naseer.
Officer '1661' wore a beard, glasses and a wig to protect his identity.
Speaking in a Northern accent, he told the court in Brooklyn how he trailed Naseer in Liverpool during late March and early April.
Prosecutors claim Naseer planned to blow up the Arndale Shopping Centre in a bid to carry out a "second 9/11".
He is also accused of planning terorr attacks in New York and Copenhagen but denies the charges against him.
After a most unusual trial in Brooklyn, jurors now must decide whether Abid Naseer plotted to blow up a Manchester shopping centre.
Osama bin Laden was sent letters updating him of the progress of terror cells in the UK, New York and Copenhagen, a US court has heard.
A man charged with drawing up plans to carry out terror attacks in Manchester, New York and Copenhagen is due to go on trial in the US.