A series of bomb explosions rocked London on Thursday 7 July.
More than 50 people were killed and hundreds wounded.
But what do we know about the situation and who might have been responsible?
What caused the explosions?
Three bombs went off almost at the same time on Underground trains during the morning rush hour. The final explosion was on a double-decker bus.
Do the police know who the bombers are?
The police believe that three of the four suspected bombers died in the blasts. The fourth is missing, but police think he may have died in the wreckage of the Tube train blast between Kings Cross and Russell Square.
Detectives have also arrested one man in West Yorkshire following raids on six addresses. It's thought the man is a relative of one of the suspects.
On Tuesday police carried out a controlled explosion on a car at Luton train station which is thought to have been used by the bombers. The car contained explosives, and is now being examined by the police.
Did the bombers work alone?
Police are trying to find out whether the four suspects are linked to a bigger terrorist group, or whether they were taking orders from anyone else.
Shortly after the blasts the terrorist group al-Qaeda claimed it was responsible for the attacks on a website, but this has not been confirmed.
It claimed it had carried out the attacks to get revenge on Britain for "massacres" in Iraq and Afghanistan.
Why did the bombers strike when they did?
It is possible that the bombers struck because 7 July was the day when the G8 summit started in Scotland. This was the high-profile meeting of eight world leaders, including British Prime Minister Tony Blair.
Did the emergency services cope?
Police, ambulances and fire crews all got to the scenes of the explosions quickly. They worked together to treat injured people and take some victims to hospital. Fire crews also worked to free passengers who were trapped in the underground.
Security in London was massively increased after the 11 September terrorist attacks in the USA in 2001. Police and fire and rescue crews have taken part in mock exercises to test how they would respond to an incident of this kind.
What is the advice for people in London?
The advice from the government and the emergency services is that people should carry on with their daily lives as normally as possible.
There is a huge police investigation into who was behind the blasts and the government is looking at what can be done to prevent more attacks.