Saying a second attack could be imminent, British Prime Minister Theresa May raised the United Kingdom terror threat level to ‘critical.'

It is the first time in 10 years England has raised the threat level to its maximum level.

The decision comes in the wake of the suicide bomber attack at Manchester Arena on Monday night, following a concert by pop star Ariana Grande. 22 people were killed and 59 injured.

While British officials are currently unable to confirm whether suspected bomber Salman Abedi acted alone or was part of a wider network of cells who could instigate more incidents, May raised the terror level and deployed the military to assist police and protect other English sites.

Manchester itself has some notable tourist attractions, including the famed Old Trafford soccer stadium, the National Football (soccer) Museum and Manchester Cathedral.

“It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack,” May said at an evening press conference.

May added that she felt the measures were a proportionate and sensible response but noted she did not want the public to feel “unduly alarmed.”

How this affects tourism in Europe, and England specifically, remains to be seen. According to CNN, overseas visitors to the U.K. rose 18 percent in the first three months of 2017 to 8 million.

CNN also noted that Euromonitor downgraded its forecast for visitor numbers to England to 4.9 percent from 5.1 percent—or about 100,000 fewer travelers visiting the U.K.

"This kind of incident has a big emotional impact on people, making [them] feel that the world is more dangerous and not as welcoming," Patrick Surry, chief data scientist at the travel firm Hopper, told CNN.

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