London attacker acted alone: Police
Terrorist Khalid Masood who killed four persons in an attack at the Westminster Bridge here, acted alone and there is no information suggesting further attacks are planned, police said.
"We must all accept that there is a possibility we will never understand why he did this," BBC quoted Deputy assistant commissioner Neil Basu as saying.
About 50 persons were injured when Masood drove his car into a crowd and then stabbed police officer Keith Palmer at the grounds of Parliament.
The family of Palmer said his bravery will be remembered.
MP Tobias Ellwood, one among those who tried to save Palmer's life, said he was "heartbroken" that his efforts were not successful.
Detectives confirmed the attack was over within 82 seconds.
Basu said: "We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned.
"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.
"Nevertheless, we are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him.
"If the latter proves to be the case, they will face justice," Basu said.
He urged those who knew Masood to speak to police.
In a statement released on Saturday, Palmer's family addressed those who tried to save his life.
"There was nothing more you could have done," it said.
"You did your best and we are just grateful he was not alone.
According to the BBC, the family said they had been "overwhelmed by the love and support" shown for Palmer and praised the support from police.
Ellwood in a statement said: "I am heartbroken that I could not do more for Palmer who gave his life in holding the line against terrorism and defending democracy."
The other victims were Aysha Frade, who was in her 40s and worked at a London sixth-form college, US tourist Kurt Cochran, 54, and retired window cleaner Leslie Rhodes, 75, from south London.
A 58-year-old man, who was arrested in Birmingham the morning after the attack, remains in custody.
A 32-year-old woman who was arrested in Manchester, remains on bail, police said.
Eleven people were initially arrested over the incident. Nine have been released without charge.
Police said Masood, 52, who had previous criminal convictions but none for terrorism, had used a number of aliases.
At birth, he was registered in Kent as Adrian Elms, but later took his stepfather's name becoming Adrian Ajao in childhood.
In the early 2000s, he was convicted of causing grievous bodily harm after slashing a man across the face with a knife.