A 52-year-old man with a long criminal record who been had been investigated for ties to violent extremism carried out the deadly attack outside the British Parliament, the authorities announced on Thursday, as the Islamic State claimed responsibility for the assault.
Details about the man, Khalid Masood, a native of England who recently lived in the city of Birmingham, emerged as the government worked to project normalcy and calm nerves the day after the attack, which took the lives of a police officer, a British schoolteacher, an American tourist and a 75-year-old man, and injured more than 40 people.
“Yesterday, an act of terrorism tried to silence our democracy,” Prime Minister Theresa May told Parliament, addressing colleagues who a day earlier had been placed on lockdown. “We are not afraid, and our resolve will never waver in the face of terrorism.” She called the violence “an attack on free people everywhere.”
Parliament observed a minute of silence for the victims on Thursday morning, while crowds gathered at Trafalgar Square in the evening for a memorial vigil. Flags flew at half-staff above the headquarters of the Metropolitan Police and Parliament. Queen Elizabeth II expressed sympathy for the victims.
At the United Nations, where the Security Council also observed a minute of silence, Britain’s foreign secretary, Boris Johnson, said, “The world is united to defeat the people who launched this attack and to defeat their bankrupt and odious ideology.” He added: “Our values are superior. Our view of the world is better and more generous and our will is stronger.”
But even as the British capital returned fairly quickly to its daily rhythms, and as Parliament resumed business — starting with a debate on trade policy — police officers were trying to learn about Mr. Masood and whether they had missed signs of his radicalization. He was born on Dec. 25, 1964, in Kent, in southeastern England, and recently lived near Birmingham, historically known for its automotive industry and now home to many South Asian and Caribbean immigrants and their children. It was there, in the Spring Hill neighborhood, that Mr. Masood rented from an Enterprise branch the Hyundai Tucson that he used in the attack.
Mr. Masood had a record of convictions, stretching from 1983 to 2003, for assault, weapons possession and violations of public order. But he was not the subject of any current investigation, and “there was no prior intelligence about his intent to mount a terrorist attack,” the London police said.
In remarks to lawmakers, before the police identified Mr. Masood, Mrs. May said the attacker was “a peripheral figure” whom MI5, Britain’s domestic counterintelligence agency, had examined for links to violent extremism. She added that he was not “part of the current intelligence picture,” that “there was no prior evidence of his intent or of the plot” and that “our working assumption is that the attacker was inspired by Islamist ideology.”
The authorities emphasized that they thought the assailant had acted alone, and that they did not expect any further attacks; Mrs. May said the nation’s threat level would remain “severe,” meaning an attack was likely, and would not be raised to “critical,” which is used to signal an imminent attack.