As police race to identify what motivated a 52-year-old British-born father to carry out Wednesday's attack at the Houses of Parliament in Westminster, a debate is simmering over issues of identity, religion and immigration -- hot topics in the wake of the vote to leave the European Union.
With police and press helicopters still buzzing overhead, thousands of Londoners gathered Thursday night in Trafalgar Square to grieve for the victims but also to express determination that life in the city will carry on as normal.
Musharaf Ahmed was one of hundreds of Muslims attending the vigil.
"These attacks — they don't have any space in Islam," he said. "Islam teaches the opposite. Islam teaches peace. The meaning of Islam is peace."
The attacker, Khalid Masood, was a Muslim convert, born and raised just outside London under the name Adrian Russell Elms.
Masood, a father of three, had previous convictions for violence but no history of terrorism.
The Islamic State group claimed online that its propaganda inspired the attack. The head of the London Metropolitan Police Counterterrorism unit Mark Rowley said Friday major questions remain unanswered.
"What led him to radicalize?" he asked. "Was it through influences in our community, influences from overseas or through online propaganda?"
Those questions echo the same deep concerns in the wake of the London bombings in July 2005 also carried out by British citizens. Who is to blame? The state, or the community? Professor Lee Marsden of the University of East Anglia has studied motivating factors behind past terror incidents.
"I think it's very easy to try to apportion blame," Marsden said. "But the reality is with lone-wolf attacks, or when people are below the radar, these type of events can occur."
The Westminster attack came at a volatile moment. Populist, anti-immigrant groups are energized by Britain's exit from the European Union. Within hours of the attack, leaders of the far right UK Independence Party blamed immigration, even though the attacker was born in the UK.
"Groups which are in mainstream political life, particularly on the right wing of political parties, will use this event to try to pursue an anti-immigrant agenda," Marsden said.
"As we saw in Brexit," he added, "a lot of the debate around immigration was not specifically around Eastern European immigration which is clearly the main result of the European Union, but actually an opportunity to attack ethnic minorities within the country."
Hayyan Bhaba, an adviser to the government on extremism, said it is vital to break down the divisions in British society. Engaging in conversation "between a lot of the frustrated communities" and having a positive dialogue can lead to "common ground," he added.
Analysts say those divisions extend beyond Britain and across Europe as the continent struggles with issues of immigration and integration.
Polish Prime Minister Beata Szydlo drew a link Thursday between the London attack and European Union migrant policy, claiming the assault vindicated Warsaw's refusal to take in refugees.