 -- ISIS has claimed responsibility for the attack that killed seven people in London on Saturday night.

According to the SITE Intelligence Group, the Islamic State’s official Amaq News Agency posted a message on Telegram on Sunday claiming that Saturday’s vehicle and stabbing attack was carried out by ISIS fighters.

“Security source to 'Amaq Agency: A detachment of Islamic State fighters carried out the London attacks yesterday,” the message reads.

Counterterrorism sources tell ABC News there is evidence suggesting the three terrorists who carried out the attack might have been waiting to strike for several months.

The trigger may have been a message posted by ISIS on Saturday that called on followers to use vehicles, guns and knives to “kill the civilians of the crusaders” during the holy month of Ramadan.

“They usually use these messages to simply inspire people to carry out attacks whenever the opportunity arises,” said Matt Olsen, the former director of the National Counterterrorism Center and an ABC News contributor. “So, really, the real goal is to motivate as many people as possible in the hopes that somebody will carry out an attack just like this.”

Police moved quickly into a working-class neighborhood in East London today and arrested 12 people, many of them connected to the dead terrorist who wore a fake bomb vest during the attack. It was a strategy likely intended not only to cause panic but also to guarantee and police response that would lead to martyrdom.

“I am not surprised that when faced with what they must have feared were three suicide bombers, the officers fired an unprecedented number of rounds to be completely confident they had neutralized the threat that those men posed.” Rowley said.

Neighbors today told ABC News the dead terrorist was a man of Pakistani descent, the father of a toddler, with his wife expecting. Others said he was known for radical views and that he recently complained that his local mosque was not devout enough.

“He said it was not a good mosque,” one neighbor said. “It was not following the Muslim religion categories.”

The BBC tonight broadcast an interview with an anonymous neighbor who said he was so concerned about the man he called authorities. “I did my bit,” he said. “I know a lot of people did, but the authorities did not do their bit.”

According to Mark Rowley, a senior officer in London’s Metropolitan Police Service, eight officers fired upwards of 50 rounds at the attackers.

With more than 3,000 potential terrorists on British watch lists, authorities say they simply cannot track every suspect or stop every attack.

“There are 500 ongoing active investigations the Security Services are carrying out, and that covers the three thousand people of interest plus others that they may not want to add into the list while they’re still subject to these ongoing investigations,” Michael Clarke, a UK counterterrorism expert, told ABC News. “There have been 13 or 14 plots that have been foiled in the last 12 months.”

Police say they already know a lot about the plot and the plotters and the ongoing investigation is focused on uncovering any potential network that could have aided the terrorists in their attack.

“Work is ongoing to understand more about them, about their connections and about whether they were assisted or supported by anyone else,” Rowley said.

So far, investigators do not see a connection between this attack and the vehicle attack in March on the Westminster bridge in London or the bomb attack two weeks ago in Manchester, but this cluster of seemingly independents suggests a broader movement that has been building for some time.

“It tell us that this wave out there is beginning to break against Britain,” Clarke said, “and that we’ve probably got to be prepared for more.

ABC News’ James Gordon Meek, Brian Epstein, Rhonda Schwartz, Margaret Katcher and Erin Galloway contributed to this story.