Police chiefs have questioned why an Al-Qaida terrorist was not prosecuted in the UK after planning "another 9/11" on Britain's streets.
Pakistani-born Abid Naseer, 28, was convicted in a New York court of plotting attacks on both sides of the Atlantic, including one in Manchester.
But the Crown Prosecution Service have been criticised for not charging him when he was arrested in the UK in 2009.
Tony Lloyd, police and crime commissioner for Greater Manchester, said: "The reality is that, had the Americans not acted, a dangerous man who was intent on causing death and destruction here could potentially still be walking our streets.
"This is deeply worrying and I will be raising this issue with the Home Secretary because we need real assurances that whatever went wrong here is never repeated. The public will want to know why he wasn't brought to trial here."
The CPS say there was not enough evidence to prosecute Naseer in the UK.
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.