Belgian police inspect an apartment in Verviers, Belgium, on Jan. 15, 2015. Police killed two men who opened fire on them during one of about a dozen raids on Thursday against an Islamist group that federal prosecutors said was about to launch "terrorist attacks on a grand scale." Reuters

The anti-terrorism raids in Belgium over the last two days set off a wave of security sweeps across Europe Friday as police officers worked to prevent another attack like the ones in Paris last week that killed 17 people. The uptick in security sweeps, experts say, signals Europe’s heightened fear that perhaps more violence is on the way, and that Islamist extremists have shifted their focus from gaining ground in Iraq and Syria, to attacking the West.

U.K. Prime Minister David Cameron warned in a Friday press briefing that an attack in his country was “highly likely.” Law enforcement officers in France, Germany and Belgium conducted raids and arrested more than 24 suspects linked to terrorism.

“It’s not a coincidence that the Parisian attacks were followed by a crackdown,” said Karen Greenberg, director of the National Security Center at Fordham Law. “I think they have been paying attention to these people for a long time and were worried about it but did not know exactly what turn it would take.”

Clint Watts, a former counterterrorism official, said that based on his research, the terrorism threat in Europe and the recent crackdown by authorities is a result of jihadist groups like the Islamic State losing ground in Iraq and Syria.

"It's like a yo-yo: What goes down on one side goes up on the other," he said. "ISIS is losing ground in Syria, which means they are resorting to more conventional methods at home in Europe."

The anti-terror raids in Belgium began in the early evening hours Thursday. Two Islamist jihadists who had returned to Belgium after fighting in Syria were killed, and a third was taken into custody when special police units raided 10 addresses in the cities of Brussels and Verviers. Authorities questioned 13 suspects and asked France to question two people on its behalf Friday.

"These raids are about reassuring the public as well as dissuading people that are in some process of a terrorist plot,” said Greenberg. “I think that it is a show of a sense of commitment to taking care of this, to pay attention to this going forward.”

Residents in Belgium told International Business Times that though they were relieved security forces had stopped the suspects from carrying out a terrorist attack, they were convinced more jihadists were still plotting.

According to a study published by the International Center for the Study of Radicalization, Belgium has the highest per capita ratio of locally born jihadists fighting in Syria and Iraq of any Western European country. The study said there were at least 300 fighters who had returned to the country. Forty six of those jihadists were linked to the group known as Sharia4Belgium, which is currently on trial in Antwerp.

"Reading about the raids didn't come as a shock. It didn't surprise me that all of a sudden, jihadists were localized in Belgium or that they were planning an imminent attack," Brechje Moerman, a resident of Brussels, told IBTimes. "It remains my feeling that some things can easily pass under the radar."

In a joint press conference with President Barack Obama Friday, Cameron said authorities in the U.K. were on alert and the terror level threat there was "severe," which means an attack is "highly likely." In July, U.S. Attorney General Eric Holder said in a speech in Oslo that European countries should ramp up their anti-terrorism tactics, and urged them to adopt American-style counterterrorism laws. (For example, the FBI tracks, through surveillance on social media and phone calls, people who express interest in joining terrorist groups or attacking America.)

“In the face of a threat so grave, we cannot afford to be passive,” Holder said. “Rather, we need the benefit of investigative and prosecutorial tools that allow us to be pre-emptive in our approach to confronting this problem.”

On Friday Obama said that while the U.S. does have terrorism concerns, the country's "Muslim population feel themselves to be Americans." He said, "There is this incredible process of immigration and assimilation that is part of our tradition. That is probably our greatest strength."