Saturday's rampage followed a suicide bombing that killed 22 adults and children at a pop concert in Manchester two weeks ago, and an attack in March when five people died after a vehicle was driven into pedestrians on London's Westminster Bridge.
12 people have since been released without charge.
"The ceramic pink knives were found on or nearby the three attackers - Khuram Shazad Butt, Rachid Redouane and Youssef Zaghba - after police shot them dead at approximately 22.16hrs on Saturday, 3 June", according to a police statement. The identity of the third attacker has also emerged, as has the fact that not only was the United Kingdom warned about him, but also the possibility that had there been action on that intelligence, the attacks could have been avoided.
The police there had found "extremist material" on his mobile phone.
Britain's failure to keep tabs on London attacker Youssef Zaghba despite being warned about him by Italian authorities was understandable, Italy's top policeman said today. One approach under consideration is to create systems for counter-terrorism agencies to share information about former subjects of interest with local police not now involved in counter-terrorism work.
Collina said she understood many British imams want to send a strong signal and have said they are not prepared to give her son a Muslim funeral.
Police shot and killed the three attackers within eight minutes of receiving an emergency call, police said.
Butt and Redouane, who also used the name Rachid Elkhdar, lived in Barking, while Zaghba is reported to have worked at a restaurant in London.
During the confrontation, passers-by can be seen throwing objects at the attackers before the last victim is shown stumbling away.
Police in Ireland on Tuesday arrested a man as they pursued a strand of the investigation into the weekend attack, they told CNN. It was the third such deadly attack in Britain in three months. Her family said she had died in her fiancé's arms after being struck by the attackers' speeding van.
Butt did not fall into that category when they last investigated him.
He had notably featured in a Channel 4 television documentary entitled "The Jihadis Next Door" and, according to the British media, numerous people alarmed by his views had gone to the authorities.
Mr. Khan said: "Not unreasonably, these questions are being asked".
Khan says "our resilience, unity and defiance of those evil individuals who seek to harm us and destroy our way of life will never change".
The family of an off-duty police officer, Charlie Guenigault, who has been hailed as a hero for running toward the attackers, praised his bravery and said he would likely do the same again if faced with the same situation.
Detectives from London police's counterterrorism command raided properties in east London on Thursday, detaining three people on suspicion of the commission, preparation or instigation of terrorism offenses. Plots involving the group's followers include an attack on a Tel Aviv nightclub in 2003, an al Qaeda orchestrated plot to bomb the United Kingdom with fertilizer bombs in 2004, a plot to bomb a territorial army center in Luton in 2012 and the murder of British soldier Lee Rigby in east London in 2013. Two people believed to have documents linked to Redouane were arrested in Ireland, though only one remained in custody Wednesday.
British police said Zaghba had not been a subject of interest for them or the domestic intelligence agency MI5 before he and two others launched their attack.
There were also office chairs, a suitcase and two bags of gravel which Haydon said might have been to add weight or to act as a cover story for their activities to friends and family.
Labour leader Jeremy Corbyn expressed concern that the police and security services lacked the resources they needed to deal with the terrorist threat.