UPDATE: 4 dead in London attack including 1 police officer, attacker; 20 injured

The head of counterterrorism at London's Metropolitan Police says four people have died in the terror incident in London, including an attacker and a police officer.

Mark Rowley says some 20 people have been wounded and parliament was locked down.

A search is underway to make certain no other attackers are in the area though police believe there was only one attacker.

Rowley said the dead policeman was one of the armed officers who guard parliament.

The other victims were on Westminster Bridge.

Rowley says ``We are satisfied at this stage that it looks like there was only one attacker. But it would be foolish to be overconfident early on.''