Here's what you need to know.
Police say they believe Masood drove a rental car into pedestrians walking on Westminster Bridge, near the British Parliament. He then fatally stabbed a police officer outside the Parliament before being shot and killed, officials said. Including Masood, four people were killed and around 40 are thought to be injured.
Who Was The Attacker?
Masood, 52, was a British man born as Adrian Russell Ajao in Kent, a county in Southeast England, police said. Prior to Wednesday's attack he had been living in the central city of Birmingham. He was known to police and had multiple previous convictions including for inflicting grievous bodily harm and for possessing offensive weapons, according to a police statement.
Masood, who also went by other aliases, was last convicted in 2003 for possessing a knife, the police statement said. He had no terrorism convictions and police said there was "no prior intelligence" about his intentions to commit an attack, the Associated Press reports.
British Prime Minister Theresa May earlier told lawmakers that Masood was "not part of the current intelligence picture."
Did he act alone?
May told lawmakers on Thursday that it was believed Masood "acted alone." ISIS have claimed responsibility for the attack, according to the Associated Press. Metropolitan Police counterterrorism chief Mark Rowley had said earlier that the attack was "inspired by international terrorism."
"We remain keen to hear from anyone who knew Khalid Masood well, understands who his associates were and can provide us with information about places he has recently visited," Rowley said.