The British Police believe that UK Parliament attacker Khalid Masood carried out the attack alone, BBC reported. “There is a possibility we will never understand why he did this,” Deputy Assistant Commissioner Neil Basu said. The incident on March 22 left four people dead.

“Even if he acted alone in the preparation, we need to establish with absolute clarity why he did these unspeakable acts to bring reassurance to Londoners, and to provide answers and closure for the families of those killed and the victims and survivors of this atrocity.” He also urged people who knew Masood or had interacted with him in the recent past to come forward and help the police in the investigation. There is no intelligence suggesting further attacks, Basu said.

As many as 11 people were detained in connection with the case, of which nine were released later. A 58-year-old man arrested from Birmingham is still in police custody, while a woman arrested from Manchester has secured bail, the Metropolitan police told BBC.

The assailant had mowed people down with his car on the Westminster Bridge before heading towards the Parliament building. At least 40 people were injured in the incident. Masood was eventually shot dead, though he managed to stab a police officer to death before that.

During the investigations, it was found that Masood was born in Kent and had been convicted for assault and possession of lethal weapons between 1983 and 2003. His former neighbours in Birmingham said Masood came across as a “nice guy”.

The Islamic State group had claimed responsibility for the attacks on Thursday, saying one of its “soldiers” had carried it out. Its affiliated news agency Aamaq had said that the assailant had carried out the attack the avenge the civilian deaths at the hands of the US-led coalition forces fighting the extremist group in West Asia.