Police identify British-born attacker, victim count rises to four

• Letter bomb at IMF's Paris office injures employee
• UK parliament attacker British-born, had been investigated over extremism concerns
• Iraqi forces fight IS in Mosul as bomb blasts hit Tikrit wedding
• Suicide bomb in market in Somalia capital kills 39

The attacker who plowed a car through a throng of pedestrians and then stabbed a policeman outside Britain's parliament was named on Thursday as Khalid Masood, a British-born man who was once investigated by MI5 intelligence officers over concerns about violent extremism.

The death toll from Wednesday's attack on the heart of Britain's democracy grew to five as police said one of the injured, a 75-year-man, had died in hospital after his life support was withdrawn.

That number included Masood, 52, who was shot dead by police.

Police work at Carriage Gate outside the Houses of Parliament. REUTERS/Neil Hall

The attack was the deadliest in Britain since 2005, when 52 people were killed by Islamist suicide bombers on London's public transport system.

It followed a series of Islamist militant operations that have killed some 280 people in France, Belgium and Germany in just over two years, and marked the third occasion a lone attacker has used a vehicle as a weapon.

Islamic State claimed responsibility in a statement issued by its Amaq news agency, but did not name Masood and gave no details. It was not clear whether the attacker was directly connected to the jihadist group.

Police said Masood was born in the county of Kent in southeast England and was most recently living in the West Midlands region of central England.

"Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," the Metropolitan Police said in a statement.

"However, he was known to police and has a range of previous convictions for assaults, including GBH (grievous bodily harm), possession of offensive weapons and public order offences."

Prime Minister Theresa May told parliament the attacker had once been investigated by the MI5 intelligence agency over concerns about violent extremism, but had been a peripheral figure.

A U.S government source said Masood had associates with an interest in joining jihadist groups abroad, but there was no evidence he had done so himself.

"The people he was hanging out with did include people suspected of having an interest in traveling to join jihadi groups overseas but the attacker himself never did so," the source said.