5:47 a.m. ET June 4, 2017   The randomness of the three terrorist attacks in Britain since March 22 puts officials around the world on edge, making it difficult to anticipate a next strike.

In the United States, New York Mayor Bill de Blasio said Sunday that his city was on high alert, with critical response units in "particularly prominent" locations.

The Islamic State, also known as ISIS, claimed responsibility for Saturday night's attack in London that left seven people dead and wounded dozens. ISIS also claimed responsibility for the previous two British attacks.

British Prime Minister Theresa May said while the recent attacks were not directly linked, “terrorism breeds terrorism.”

“We can’t say there’s an uptick in focus on England, because what we’re looking at now is a micro trend,” said Michael Pregent, an adjunct fellow at the Hudson Institute in Washington. “These were three different attacks.”

Saturday night's attack began when a van plowed into pedestrians on London Bridge then stopped in Borough Market, where three men got out and stabbed people. The three attackers were shot dead by police. On May 22, a suicide bomber at the Ariana Grande concert in Manchester, England, killed 22 people and injured 59. The bomber was identified as British-born Salman Abedi, 23.

On March 22, Khalid Masood, 52, rammed his car into pedestrians on Westminster Bridge, killing five people, and he then fatally stabbed a policeman outside Parliament. Masood was shot dead by police. The Islamic State, also known as ISIS, claimed Masood as a "soldier,” though police later said he had no links al-Qaeda, ISIS or any other extremist group.

The attacks each show a different level of sophistication, Pregent said. Masood used a low-tech knife, while Abadi used a well-made bomb in a vest and had recently returned from Libya, where his relatives have ties to the Islamic State, according to investigators.

The London Bridge attack involved “three individuals who agreed to kill and be killed, all three wearing (fake) suicide vests,” Pregent said. “That would indicate more of a cell.”

The differences among the attacks indicates they probably are not directly related, Pregent said.

Mia Bloom, a professor of communications and Middle East studies at Georgia State University, said terrorist groups have been wanting to get Muslims “off the fence” in European countries, including Britain, which they contend have an anti-Muslim bias. And that push has intensified since last month's attack at the concert, Bloom said.

“In ISIS chat rooms there has been a strong push to attack,” Bloom said. “Ever since Manchester, there have been memes about attacking in London. They actually had instructions about ramming with trucks.”

Bloom said the recent online messages have been in line with the teachings of American-Yemeni cleric Anwar al-Awlaki who taught English-speaking followers that if they could not join the war in the Middle East they should act locally. Awlaki was killed in Yemen in a U.S. drone strike in 2011.

As U.S.-led forces continue to take ISIS territory in Iraq and Syria, the militants “need to project a sense of still being in control,” Bloom said.

Andrew Parker, head of the British domestic intelligence agency MI5, warned in 2015 that several factors were contributing to an “unprecedented level of threat” to British citizens from ISIS, largely from Syria's ongoing civil war.

Britain has a large well of 23,000 radicalized Muslims, including 3,000 judged to pose an imminent threat, intelligence officials told the British Sunday Times in a May 27 report. Estimates of Britons who traveled to fight with radical groups in Syria, range from 800 to 2,000, according to media reports. It’s unclear how many of them had returned.

The threat is caused by three levels of influence by groups like ISIS, said Thomas Joscelyn, a terrorism analyst at the Foundation for Defense of Democracies, a think tank in Washington.

The group provides inspiration through its media operations on websites, YouTube, private chat rooms and messaging apps that allow ISIS to connect with followers who have no direct contact with the group in Britain, Europe or elsewhere.

ISIS also provides online tutorials on building bombs or using cars and knives to kill people. Three such “remote control” attacks occurred in Germany in 2016: a knife and ax attack on a train in Würzburg, a suicide bombing in Ansbach, both in July, and a stabbing attack on a police officer in Hanover in February.

And it plans and executes attacks from start to finish, like the Paris attack in November 2015, which killed 130 people in a series of coordinated attacks around the French capital.

“It isn't surprising that ISIS has claimed responsibility” or that attacks are happening in Britain, Joscelyn said. “They’ve been putting effort into this for the better part of three years. We don’t know the precise reason we’re seeing successful plots now.”