The attack lasted 82 seconds.
At 14:40 GMT on Wednesday 22 March, a man identified by police as Khalid Masooddrove a car that he had hired from a depot in Birmingham over Westminster Bridge, near the Houses of Parliament.
One witness said he sped up, mounted the pavement, and began hitting pedestrians indiscriminately.
The BBC has learned that police believe Khalid was driving up to 76mph on the bridge.
- 14:40:08 – the car that Masood was driving over Westminster Bridge first mounted the pavement on the northbound side
- 14:40:38 – after continuing towards Bridge Street along both the footpath and road, Masood crashes into the perimeter fence of the Palace of Westminster
- 14:40:59 – the first 999 call was made to the Met police reporting the incident
- 14:41:30 – Masood left the vehicle and was shot by a police firearms officer inside the Palace of Westminster
Two people, Aysha Frade, a British national, and US tourist Kurt Cochran, were killed and dozens more were injured. A 75-year-old man, Leslie Rhodes from Clapham in south London, died from his injuries the next day.
The car then crashed into railings outside the Houses of Parliament.
Masood, armed with a knife, left his car and ran towards Parliament, where he was confronted by police.
PC Keith Palmer – who was wearing a protective vest but was not armed – was stabbed and killed. Masood was then shot dead by armed officers.
Reports that he was shot by a member of Defence Secretary Michael Fallon’s close protection team – rather than one of Parliament’s armed police – have not been confirmed.
Detectives have confirmed that the attack was over within 82 seconds.
Who was the attacker?
Khalid Masood, 52, was born in Kent on Christmas Day 1964. Before Wednesday’s attack he is believed to have been living in the West Midlands.
He was entered onto the birth registry in the Dartford district of Kent as Adrian Russell Elms, in the weeks after his birth.
However, two years later his mother married a man with the name Ajao and, as a child, Masood was known as Adrian Russell Ajao. He later used both Ajao and Elms until he converted to Islam and became Masood.
He had a criminal past but his most recent conviction, for possessing a knife, was back in December 2003 and he had never been convicted of a terrorism offence.
Masood spent a number of years working in Saudi Arabia teaching English as a second language at the General Authority of Civil Aviation in the port city of Jeddah, the BBC understands.
Masood, who stayed in a hotel in Brighton on the night before the attack, gave his occupation as “teacher” when hiring the car with which he carried out the attack.
The Department for Education said it had no record of him having worked as a qualified teacher in English state schools.
He is believed to have had at least three children.
Prime Minister Theresa May told MPs that while Masood had been “once investigated in relation to concerns about violent extremism” he was a “peripheral figure” and “not part of the current intelligence picture”.
Who were the victims?
The police officer who died was PC Keith Palmer, 48, of the parliamentary and diplomatic protection command. He had 15 years’ service, was married and had a five-year-old daughter. He had served in the Royal Artillery before joining the police.
Aysha Frade, who worked in administration at DLD College London, was killed after being hit by the attacker’s car before it reached Parliament.
It is thought she was 43 and a married mother of two young daughters.
A third victim was Kurt Cochran, from Utah, who was in London to celebrate his 25th wedding anniversary. His wife Melissa was seriously injured but is said by her family to be “steadily improving”.
Leslie Rhodes, a retired window cleaner, had been receiving medical treatment in hospital following the attack. His life support was withdrawn on Thursday evening.
Who are the injured?
At least 50 people from at least 12 countries were injured in the attack, 31 of whom needed hospital treatment.
Thirteen of them remain in hospital.
Among those injured were three police officers who had been walking across Westminster Bridge on their way back from a commendation ceremony.
One of the two officers recovering in hospital has been named as PC Kristofer Aves. He is one of two described as having “very significant” injuries.
Acting Deputy Commissioner Mark Rowley said the victims represented “a real cross-section of ages”. Among them, according to the prime minister, were:
- 12 Britons
- Three French children
- Two Romanians
- Four South Koreans
- Two Greeks
- One each from Germany, Poland, the Irish Republic, China, Italy and the US
The Romanian couple are Andreea Cristea and Andrei Burnaz.
The Romanian embassy in London has confirmed that Ms Cristea was the woman who was seen falling from Westminster Bridge into the Thames during the attack, suffering multiple injuries.
Among others injured were four university students from Edge Hill University, in Ormskirk, Lancashire.
What do we know about the police investigation?
Eleven people were arrested in the 48 hours following the attack, but nine have since been released with no further police action. Several addresses have been raided in London and Birmingham in connection with the attack.
The Met Police said on Saturday evening that one man – a 58-year-old who was arrested in Birmingham on the morning after the attack – remained in custody.
Another women, aged 32, who was arrested in Manchester, remains on police bail while police continue their inquiries, police said.
On Sunday evening, police said a 12th arrest – of a 30-year-old man – had been made in Birmingham.
Acting Deputy Commissioner Mark Rowley said 2,700 items had been seized, including “massive amounts” of computer data.
Masood’s phone is understood to have connected with WhatsApp two minutes before the attack, but the messaging app’s encryption system means police may not know what if anything was communicated.
The Hyundai driven by Masood had been hired in Spring Hill, Birmingham, from a branch of Enterprise.
Police have been contacted by about 3,500 witnesses – 1,000 who were on Westminster Bridge and 2,500 who were within the Parliamentary estate at the time.
Hundreds of videos and pictures have been uploaded to the NPCC website, Mr Rowley added.
On 25 March, Deputy Assistant Commissioner Neil Basu said detectives believed Masood had acted alone and that there was no information or intelligence to suggest further attacks were planned.
“There is a possibility we will never understand why he did this,” he added.
What security has been put in place?
In London, the number of armed officers remains at “near double strength”, Mr Rowley said, while across the UK there are “up to a third” more armed officers on duty.
He added: “Our current arrangements have been developed with Parliament over many years and are designed to provide access to the seat of our government, balanced carefully with security that is proportionate but not overly intrusive.
“Of course, after an incident like this, as would be expected, my team will work with parliamentary authorities to assess whether a different tone or balance is necessary.”
The UK’s terror threat level has been set at “severe” – meaning an attack is highly likely – for some time and this would not change, the prime minister said.