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.
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.
Police have already named the other two attackers as Khuram Butt and Rachid Redouance.
After the third attack in less than three months, Prime Minister Theresa May said Thursday's election would go ahead.
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.
British authorities have not found evidence linking Khalid Masood to terror groups.
Authorities won't confirm nor deny reports that a gate was left undefended while a policeman was being helped.
Official says Khalid Masood's motivation may never be known.
Just as we oppose those who bring terror to our streets, we should not allow others to exploit terrorism for evil either.
The U.K. has been on 'severe alert' since 2014, meaning an attack is highly likely.
"The problem we have nowadays is this form of low-cost terrorism," the National Front leader said.