One of the men seen on video subduing the knife-wielding suspect has been identified as James Ford, a convicted murderer on day release who was attending the same prison education conference as the attacker.
A 21-year-old man was arrested Saturday evening in Hounslow, west London.
If ISIS was behind Friday's bombing of the London metro, it would be the group's fourth attack in the U.K. this year.
Without the oxygen of publicity, the attacker is just that: a killer with a car and a knife. The lone wolf needs the pack as much as the pack needs the lone wolf.
Monday's attack targeted worshippers from an institution with a proud history of fighting violent extremism.
Figures released by Mayor of London Sadiq Khan show as many as 20 incidents per day.
Khuram Butt and Rachid Redouane were "warned about their conduct" after wrestling in the water.
The divisions in the East End community have turned it into a flashpoint between white nationalists and radical Islamists.
Trump's son claimed CNN and other media outlets staged a protest of Muslims condemning ISIS.
Police have already named the other two attackers as Khuram Butt and Rachid Redouance.
Revellers shouted and threw barstools at the three attackers, who killed seven and injured 49.
After the third attack in less than three months, Prime Minister Theresa May said Thursday's election would go ahead.
The attackers ploughed a van into pedestrians on London Bridge before running into Borough Market with knives and stabbing people at random.
The woman from Canada moved to Europe to live with her fiancé, her family said in a statement.
The president wrote a string of tweets in the aftermath of the London Bridge attack that left seven people dead.
Like the Bataclan and Manchester Arena, Borough Market was targeted for being a place of celebration and joy.
The arrests were made in connection with Saturday's attack that killed seven.
Tourists and locals spent the night in hotels after the attack that killed seven people and injured dozens.
Andreea Cristea was visiting London at the time of the attack and becomes the fifth victim to die.