Four people died in an Islamist-claimed attack outside parliament in London this week when a British-born killer rammed his car into pedestrians then stabbed a police officer before being shot dead.
Here is what we know so far about the attack, which was the deadliest in Britain since 2005 and was claimed by the Islamic State jihadist group.
- What happened? -
At around 2:40 pm (1440 GMT) on Wednesday, the attacker ploughed his car into pedestrians on Westminster Bridge, one of the city's busiest tourist spots for its views of parliament and its famous clock tower.
After crashing into the railings outside parliament, he leapt out with a large knife and ran into the courtyard where he stabbed a policeman before being shot dead by officers.
Policeman Keith Palmer, 48, died of his injuries along with three civilians who were hit by the speeding car: 43-year-old mother-of-two Aysha Frade; Kurt Cochran, an American in his 50s; and 75-year-old Leslie Rhodes from south London who died late Thursday.
At least 50 people were wounded, including tourists from 12 different countries. One person is fighting for their life and two are in critical condition.
- Who was the attacker? -
Police have identified the perpetrator as Khalid Masood, a 52-year-old Briton who was born in the southeastern county of Kent under the name Adrian Russell Ajao.
So far, police have not yet released an image of Masood although media footage of the attacker lying on a stretcher showed a bearded, burly man wearing black clothes.
He ran into trouble with the law in his late teens, chalking up a string of convictions for assault and possession of offensive weapons between 1983 to 2003, but none of them terror related.
According to The Sun tabloid, he married a Muslim woman in 2004 and a year later moved to Saudi Arabia to teach, returning in 2009.
Police said he went by "numerous aliases", including, reportedly, Adrian Elms, while reports suggest he lived all over England, including in Luton and east London.
Prime Minister Theresa May has said Masood was known to intelligence services as a "peripheral" figure some years ago but there was no warning of his intention to mount an attack.
When he hired the car, he told the Birmingham-based rental company he was a teacher, the BBC said. But an education ministry spokesman told AFP he was not a qualified teacher and had not taught in any state schools.
A former neighbour from Winson Green in Birmingham described him as "a nice guy", telling the Birmingham Mail newspaper he lived next door with his wife and a small child until Christmas and liked to work in his garden.
- Did he act alone? -
The attack was claimed by Islamic State which said Masood was one of its "soldiers" acting on a call to target countries fighting the jihadists in Iraq and Syria.
Counter-terror chief Mark Rowley said police were trying to establish whether Masood acted totally alone "or if others have encouraged, supported or directed him".
So far, police have arrested 10 people in connection with the attack. They initially arrested five men and three women "on suspicion of preparation of terrorist acts". One of the women was later released on bail.
Two more "significant arrests" were made overnight Thursday.
Police have so far searched 16 addresses, mainly in London and Birmingham, seizing 2,700 items including "massive" amounts of computer data. Five more raids are currently under way, Rowley said.
- What has been the response? -
Following the attack, May said Britain would maintain its terror threat level at "severe" although extra police have been put on the streets and armed officers stationed at the entrance to parliament.
MPs returned to work as normal on Thursday, even as forensic officers worked at the scene.
During the evening, hundreds gathered in Trafalgar Square in London with flags, flowers and messages of defiance at a vigil led by Mayor Sadiq Khan who vowed: "Londoners will never be cowed by terrorism."
- Has this happened before? -
The attack had echoes of last year's carnage in Nice and Berlin when lone attackers drove trucks into crowds of people, killing 86 people in the French Riviera city in July and 12 at a market in the German capital before Christmas.
Wednesday's attack was the deadliest in Britain since July 2005 when four suicide bombers killed 52 people on the city's transport system, in a coordinated assault by British nationals inspired by Al-Qaeda.
Since then, there have been other incidents, including the grisly murder of British soldier Lee Rigby in which two Islamic extremists hit him with a car then hacked him to death in broad daylight on a London street.
And last August, a paranoid schizophrenic tried to behead a commuter with a knife at a London Underground station in an IS-inspired attack.