A detective was reportedly still negotiating with robbery suspect Marcus Fischer when cops shot him numerous times.
The deadly stabbing attack in Dagestan follows a similar incident in Siberia that left eight people injured, according to reports.
The 18-year-old killed two people and injured six more in the southwest Finnish city of Turku.
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.
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 arrests were made in connection with Saturday's attack that killed seven.
Rob Lyon describes how he survived an attack in the heart of British democracy.
The Somali attacker had criticized the media for its portrayal of Muslims.
The man wounded up to nine people in a knife attack at a mall in central Minnesota before he was shot dead by an off-duty police officer.
Zakaria Bulhan also charged with five counts of attempted murder in Russell Square attack.
"We don't know if it's a hoax but there's no nexus to the murders," LAPD Captain Andrew Neiman said.