The investigation into Butt was started in 2015, "however, there was no intelligence to suggest that this attack was being planned and the investigation had been prioritised accordingly", Mr Rowley said.
Officers arrested him in the early hours of Wednesday in the eastern London suburb of Ilford, near Barking, where two of the assailants lived.
They said Butt, who was 27, was a Pakistan-born British national known to MI5.
The third man is 22-year-old Youssef Zaghba, an Italian-Moroccan who was registered as an Italian living overseas and whose parents had separated. The officials spoke on condition of anonymity because they were not allowed to discuss details of the case.
"I don't know what kind of madness took him to do such a madness thing and leave two lovely kids".
Reports have described Butt, who was pictured in the aftermath of the attack with canisters strapped to his body, as the ringleader of the trio.
Transport for London, which oversees the capital's transport network, has advised commuters, struggling to get to work in the driving rain, to make alternative journeys as the station will be busy.
In particular, the revelation that at least one of the attackers, Khuram Butt, was well known to security services has raised concerns that they lack the resources to prevent attacks.
Pri Goncalves said on Facebook she was the last person to see her on Saturday night.
May's Conservative Party's lead over Labour has narrowed markedly from 20 points or more when she called the election in April to a range between one and 12 points now, although the Conservatives are still widely expected to win a majority.
She said the U.K.'s national threat level remained at severe on Monday, which "means that a terrorist attack is highly likely".
A Frenchman killed in the attack worked at the bistro in Borough Market, French media reported. "But perhaps more importantly, authorities also believed that the existence of such groups would allow authorities to keep an eye on suspects, given that members were on the radar of authorities", said Shiraz Maher, deputy director of the London-based International Center for the Study of Radicalization and Political Violence.
The election campaign was suspended for several days in May, when a suicide bomber killed 22 people at a concert by USA pop singer Ariana Grande in Manchester.
It was lowered once intelligence agencies were comfortable that this wasn't the case.
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