In an attack in London on Wednesday, a lone assailant plowed a car into crowds of people gathered on Westminster Bridge before stabbing a police officer dead outside UK Parliament. The assault on the heart of Britain's capital left four people dead.
On Thursday, police announced the fourth fatality and named the attacker as 52-year-old Khalid Massod. But there is still much we don't know about the man who carried out the deadliest terror attack the UK has seen for more than a decade.
What we know
• Police named the attacker as 52-year-old Khalid Masood -- known by a number of aliases.
• Masood, who was born in Kent, was most recently living in the West Midlands, according to police.
• He has never been convicted for any terrorism offenses, according to police.
• Known to police, Masood had a range of previous convictions for assaults, including grievous bodily harm, possession of offensive weapons and public order offenses.
• His first conviction was in November 1983 for criminal damage; his last conviction was in December 2003 for possession of a knife.
• Speaking before Masood was named, Prime Minister Theresa May said the attacker was known to authorities for links to "violent extremism."
• He was investigated "some years ago" by security services, but was regarded as a "peripheral figure," May said in the House of Commons.
• He was not part of the "current intelligence picture," and authorities did not know he was about to mount an assault.
• The vehicle used in the terror attack was traced to a Birmingham car company.
• Britain's most senior counter-terror police officer said investigations were ongoing in London, Birmingham and elsewhere on Thursday.
• The attacker is believed to have acted alone, according to Mark Rowley, the lead officer in the UK for counter-terrorism policing.
• The working theory is that the attack was ISIS "inspired or copycat," a UK official told CNN.
• ISIS-affiliated news agency Amaq has claimed responsibility for the attack, calling the attacker a ''soldier of Islamic State.''
What we don't know
• Masood's movements before the attack. Authorities have searched addresses in Birmingham and elsewhere, but the attacker's whereabouts before Wednesday have not yet been confirmed.
• His motive. A UK official told CNN: "52 is interesting. Not the usual young pup profile" for an ideologically-driven attacker.
• Whether he acted alone. A number of arrests have been made in Birmingham connected with the attack, but it is not clear how those detained were associated with the attacker.
• The veracity of Amaq's claim. Just because the ISIS-linked news agency asserted the attack was carried out by one of its "soldiers" does not necessarily mean the group had any direct connections to the attacker.