LONDON - The attacker who killed three people near the British parliament before being shot dead was British-born and was once investigated by MI5 intelligence agents over concerns about violent extremism, Prime Minister Theresa May said on Thursday.Police arrested eight people at six locations in London and Birmingham in the investigation into Wednesday's lone-wolf attack that May said was inspired by a warped Islamist ideology. Forty people were injured and 29 remain in hospital, seven in critical condition. The Islamic State claimed responsibility for the attack on Thursday, but the veracity of their claims remained unconfirmed.The assailant sped across Westminster Bridge in a car, plowing into pedestrians along the way, then ran through the gates of the nearby parliament building and fatally stabbed an unarmed policeman before being shot dead. "What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism," May said in a statement to parliament."He was a peripheral figure...He was not part of the current intelligence picture. There was no prior intelligence of his intent or of the plot," she said, adding that his identity would be revealed when the investigation allowed.Westminster Bridge and the area just around parliament were still cordoned off on Thursday morning and a line of forensic investigators in light blue overalls were on their hands and knees, examining the scene where the attacker was shot.The dead were two members of the public, the stabbed policeman and the attacker.It was the worst such attack in Britain since 2005, when 52 people were killed by militant islamist suicide bombers on London's public transport system. Police had given the death toll as five but revised it down to four on Thursday.The casualties included 12 Britons, three French children, two Romanians, four South Koreans, one German, one Pole, one Chinese, one American and two Greeks, May said."We meet here, in the oldest of all parliaments, because we know that democracy and the values it entails will always prevail," she said."A terrorist came to the place where people of all nationalities and cultures gather what it means to be free and he took out his rage indiscriminately against innocent men, women and children," said May.A minute's silence was held in parliament and in front of police headquarters at New Scotland Yard at 0933 GMT, in honor of the victims -- 933 was the shoulder number on the uniform of Keith Palmer, the policeman who was stabbed to death.Unconfirmed media reports emerged Wednesday night which cite the attacker as Abu Izzadeen, a spokesman for the Islamic extremist organization Al Ghurabaa. Others, on the other hand, have claimed that Abu Izzadeen could not have been the attacker as he is currently serving a prison term for a previous offense. An ABC reporter tweeted that she heard from Izzadeen's lawyer that he is still in jail.According to The Guardian, Izzadeen, a British national, was jailed in 2008 for inciting attacks against American and British armed forces during the invasion of Iraq and funding terrorism. Izzadeen was born to a Christian Jamaican family as "Trevor Brooks", and the Guardian reported that he is believed to have been "radicalized" when he was 17, when he met Omar Bakri Muhammed and Abu Hamza. Izzadeen, along with well-known Islamic extremist Anjem Choudary, appeared together in a YouTube video published in 2015 urging Muslims not to vote in parliamentary elections as it is prohibited in Islam as shirk, an unforgivable crime. He also stated that Muslim members of Parliament were apostates. Izzadeen fervently advocated against Kufr (non-Muslim) law in the United Kingdom and cited lack of British intervention in Syria and the "Occupation of Palestine" as support for the "murder of children."