London Bridge attacker joined London Underground while beinginvestigated by terrorist police
 

Metropolitan Police assistant commissioner Mark Rowley said the
investigation into Butt had begun two years ago but "there was no
intelligence to suggest that this attack was being planned and the
investigation had been prioritised accordingly".

Butt featured in a Channel 4 documentary last year about Islamist
extremists with links to the jailed preacher Anjem Choudary.

He had also worked for London Underground for just under six months as a
trainee customer services assistant, before leaving in October last
year, Transport for London said.

http://www.bbc.co.uk/news/uk-40165646