During a training exercise for London's emergency services, a casualty is helped to leave the disused Aldwych underground train station in London, Tuesday, June 30, 2015. Hundreds of British police are taking part in a major counter-terror exercise in London, testing the abilities of emergency services to respond to a Tunisia-style attack of extremists with firearms. The exercise dubbed Operation Strong Tower has been planned for six months and is not connected to the recent attack at a north African beach resort. (Photo by Matt Dunham/AP Photo)

During a training exercise for London's emergency services, people run towards armed police officers as they emerge from the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)

People take part in a training exercise for London's emergency services outside the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)

London Metropolitan Police and emergency services take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. London police held their biggest-ever terrorism drill on Tuesday, pitting the emergency services against a group of marauding attackers, nearly 10 years since four young British Islamists killed 52 people in suicide bombings on London's transport network in July 2005. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

Emergency services run as they take part in Exercise Strong Tower with London Metropolitan Police, at the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

Member of the emergency services tend a casulty during Exercise Strong Tower at the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

An actor lies on the ground during Exercise Strong Tower at the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police and emergency services take part in Exercise Strong Tower, removing casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, removing actors as casualties from the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

London Metropolitan Police take part in Exercise Strong Tower, the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

Member of the emergency services tend a casulty during Exercise Strong Tower at the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

An actor plays an injured passer-by during Exercise Strong Tower, the scene of a mock terror attack at a disused underground station in central London, Britain June 30, 2015. (Photo by Peter Nicholls/Reuters)

During a training exercise for London's emergency services, actors hold firearms outside the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)

During a training exercise for London's emergency services, a casualty is dragged away by rescue officers from the London Fire Brigade, outside the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)

During a training exercise for London's emergency services, a casualty is helped by an armed police officer, outside the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)

During a training exercise for London's emergency services, a medic treats a casualty, outside the disused Aldwych underground train station in London, Tuesday, June 30, 2015. (Photo by Matt Dunham/AP Photo)