There were “opportunities galore” to identify that the London Bridge extremists were plotting an attack, an inquest has heard.
Gareth Patterson, the lawyer representing several victims’ families, said there was evidence the attackers had been in contact since January 2017.
Eight people died in the attack on 3 June 2017.
But investigating officer Acting Det Ch Insp Wayne Jolley denied there had been missed opportunities.
Mr Patterson told the hearing at the Old Bailey in London that “any reasonably competent investigation” had the chance to detect the planning that was going on between the three men.
It would have taken the trio a “significant period of time” for them to become close enough to trust each other with planning an attack, he said.
Khuram Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22, left 48 people injured when they attacked passsers-by near London Bridge with a van and knives, before being themselves shot dead by armed police.
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Xavier Thomas, 45, Christine Archibald, 30, Sara Zelenak, 21, Sebastien Belanger, 36, James McMullan, 32, Kirsty Boden, 28, Alexandre Pigeard, 26, and Ignacio Echeverria, 39, died in the attack, which lasted less than 10 minutes.
Mr Patterson challenged the investigating officer, pointing to repeated contact between the attackers.
The inquest heard that Zaghba had started going to Butt’s gym in January 2017 and that the two men were in telephone contact after that time.
Zaghba also visited Butt’s home and had been allowed to drive his car, the inquest heard.
In March, all three attackers were at the Ummah fitness centre in east London.
It was in the same month, Mr Patterson said, that Butt had possibly been trying to buy a gun.
There was then a barbecue at Butt’s home in May, which Redouane attended, and those two men were in contact “again and again for months”, Mr Patterson said.
The court heard that, the day after the barbecue, Redouane bought three identical knives.
“Any reasonably competent investigation should have been looking at Redouane at this stage, I would submit,” Mr Patterson said.
Khuram Butt, Rachid Redouane and Youssef Zaghba carried out the London Bridge attacks
Det Ch Insp Jolley said he did not agree that there had been missed opportunities to stop the men and said police would have been working with the intelligence they were given.
Mr Patterson sais that there was one occasion in May when all three men were at the gym “in the dead of night” and were speaking together in the street, but employed a “classic anti-surveillance technique” of leaving a telephone on the ground while they walked away and talked.
“The attack planning was there to be detected,” he suggested.
The court also heard that Zaghba had held extremist views since childhood.
He celebrated the 9/11 attacks and had the Islamic State group flags on his Facebook page, according to information from his mother.
Zaghba had also tried to flee abroad to fight for IS and had jihadist material on an SD memory card seized from him when he was stopped at an airport.
But Richard Horwell, the lawyer representing the Metropolitan Police at the inquest, asked: “In the months leading up the attack was there any evidence of any attack planning?”
Mr Jolley said: “Not that we uncovered, sir, no.”