The meeting in Amsterdam was held amid heightened fears of terror attacks on European capitals similar to the ones in November in Paris. On Sunday, the Islamic State released a video that featured the Paris attackers and included footage showing some of them executing hostages.

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"Expect a mujahid to show up to kill you," the alleged ringleader of the Nov. 13 Paris attacks, Abdelhamid Abaaoud, said in the video. Abaaoud was killed during a shootout with police in a Parisian suburb days after the attacks.

The video included general threats against Europe but also showed several landmarks in London, including the Tower Bridge -- raising concerns about possible plans by the Islamic State to strike in Britain.

Abaaoud reportedly had visited Britain early last year. Despite having returned from Syria, he entered the country on a ferry without being detected by police. Abaaoud also took photos of British landmarks. British authorities estimate that about 800 British extremists have so far gone to Syria and Iraq. Nearly 400 are believed to have returned, according to the BBC.

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British intelligence services, like authorities in other European nations, have interviewed returnees to decide who should be monitored more closely. But the London bombings in 2005 showed that it is often hard to correctly analyze who could become a threat to national security. Dozens of officers are needed to observe a suspect around the clock, which makes it necessary for intelligence services to constantly reassess their targets.

The beginning of Sunday's propaganda video showed encryption software allegedly used by the terrorists to hide from authorities. Although it has since been pointed out that the encrypted messages shown in the video are likely fakes and technically flawed, the Islamic State's focus on such tools was probably intended to raise even more alarm about the group's technical capabilities. Such fears were reflected in the Europol expert review Monday, which concluded that "the availability of secure and inherently encrypted appliances" as well as "coded language" could indeed "prevent conventional observation by security authorities."

The expert review by Europol singled out France as a main target of future attack plans. "There is every reason to expect that IS, IS inspired terrorists or another religiously inspired terrorist group will undertake a terrorist attack somewhere in Europe again," the agency wrote, using an acronym for the Islamic State.

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Bernard Cazeneuve, France's interior minister, was quoted Monday as saying that the nation's intelligence services had prevented more than 10 attacks within the past year. France recently extended its state of emergency, which was imposed after the Nov. 13 attacks.

According to Europol, the Islamic State has built a command center that oversees and plans attacks outside its core territory in Iraq and Syria with the declared purpose of conducting international terror attacks. In Europe, most of the Islamic State's terrorist cells are organized locally. Europol said that those cells could be recruiting new members in refugee camps, but the agency did not find any evidence "that terrorist travelers systematically use the flow of refugees to enter Europe unnoticed."

The Islamic State is trying to show that it is now capable of conducting a series of deadly attacks all over the world -- particularly in Europe, the report concludes.

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Europol also said that the Islamic State's rise can partially be explained through its recruitment drive in Europe. Rather than using religion as the prime driver of recruitment, social pressure among peer groups could explain the rapid increase in European recruits. Often, European Islamic State members are former criminals or have had mental problems -- features that make the recruitment process distinct from many other Islamist terror groups.

The European Police Office also warned that the Islamic State's rival terror group, al-Qaeda, remains a threat.