The families of the London Bridge victims criticised the authorities on Friday after a coroner cleared MI5 and police of failing to prevent the terror attack despite having the ringleader under investigation.

After more than seven weeks of harrowing evidence, the inquests into the deaths of the eight victims of the June 2017 attack came to an end with the conclusion that they had been unlawfully killed.

Chief coroner of England and Wales, Mark Lucraft QC, criticised the failure to erect protective barriers on the bridge to thwart a vehicle attack three months after an attack on Westminster Bridge. But he said that he was “not persuaded” that the authorities missed clues that could have helped thwart the atrocity.

After hearing his conclusions, family members took to the steps of the court to insist that they believed MI5 and police had fallen short in their duties.

Ringleader Khuram Butt, 27, had been investigated by MI5 since 2015 over concerns he wanted to stage an attack. The three terrorists drove a van into pedestrians on the bridge before stabbing people with 12-inch knives in a 10-minute rampage that scars the families to this day.

Philippe Pigeard, 51, whose son Alexandre was stabbed to death, told the Guardian: “Despite the warnings, the direct warnings, with calls from relatives of Butt, they did not put more pressure on the guy … Many of Butt’s actions are unknown by the investigators, even though they were supposed to have him under investigation.

“The question is: could this attack have been prevented? The answer is really difficult – but yes, probably. I think they could have done a better job.”

Christiane and Philippe Pesez, whose son Xavier Thomas was knocked into the River Thames by the van, said: “It is our opinion that many opportunities were missed to identify, disturb and prevent the murderous actions of three men in their preparation of this act of terror.”

Julien Belanger, whose son Sebastien was stabbed to death, echoed that claim, saying: “Many opportunities were missed to prevent this terror attack – a prior arrest of Khuram Butt’s or [to] make a pedestrianised area of London Bridge safe.”

Despite the outcome, the inquests into the van and stabbing attacks only strengthened the families’ conviction that counter-terrorism investigators should have stopped the atrocity before it happened.

During the inquests the families say the evidence showed a string of errors and missed opportunities that left Butt free to plot the attack under the noses of the authorities.

But in delivering his conclusions, Lucraft said: “My finding is that the pre-attack investigations of MI5 and SO15 [the Metropolitan police’s counter-terrorism command] were generally thorough and rigorous. On all the evidence and in the final analysis, I am not persuaded that investigative opportunities were lost which could realistically have saved the lives of those who died.”

He was more critical of the lack of protection on the bridge in advance of the incident, which came three months after a terrorist used a vehicle to kill four pedestrians on Westminster Bridge. Only after the attack on London Bridge were barriers put in place.

Lucraft said: “It is also appropriate to recognise that there were weaknesses in the systems for assessing the need for such measures and implementing them promptly, and that without such weaknesses, suitable HVM [hostile vehicle mitigation measures] may realistically have been present.”

”There was no form of physical protective security on London Bridge despite the fact that it was a location which was particularly vulnerable to a terrorist attack using a vehicle-as-a-weapon. There were weaknesses in systems for assessing the need for such measures on the bridge and implementing them promptly. Absent such weaknesses, suitable hostile vehicle mitigation measures may have been present.”

After the coroner’s conclusions, the head of counter-terrorism policing, assistant commissioner Neil Basu, said: “We are not complacent. Police, along with our security service partners, have subjected ourselves to the most painstaking and at times painful examination to determine what lessons we could learn. We have already identified and made many improvements and this work continues.”

Helen Boniface, solictor for six of the families said: “This was clearly planned as a two-stage attack. Had barriers been in place on London Bridge, they [the families] firmly believe this attack would not have taken place here at all and their loved ones would still be here today.”

The coroner criticised Butt’s family for failing to inform the authorities. The inquests heard his brother-in-law did call police but his information was mishandled and never went to teams in the counter-terrorism command and at MI5 investigating Butt. The coroner concluded: “Khuram Butt’s [other] family [members] were aware of his extremist views and behaviour, but did not report those matters to the authorities.”

Christine Delcros, the girlfriend of Xavier Thomas, another victim, said: “I believe this attack was preventable.

“I find it staggering that Butt, a well-known extremist, was allowed to work within the London transport network, to have access to and teach young children, and to rent and use a vehicle in a manner now too often encountered. I am dismayed SO15 did not pass this critical information to any of his employers.”

Facebook Twitter Pinterest From left to right: Khuram Butt, Rachid Redouane and Youssef Zaghba. Photograph: Metropolitan police/PA

The attack lasted 10 minutes and ended when armed police shot the terrorists dead. All three of them were wearing fake suicide bomb belts. Police fired 46 shots at the terrorists to end the UK’s first “marauding” attack, where perpetrators run through a crowded area.

The two other attackers were Rachid Redouane and Youssef Zaghba.

The coroner’s conclusions mark a formal end to the search for lessons from the London Bridge attack. MI5 and police will both welcome the outcome having dreaded the prospect of criticism from the coroner. But the relief is temporary, with inquests into the Manchester Arena suicide bombing next year likely to pose serious questions for investigators.