Top Intel: Manchester a Reminder That Terror Threat Is 'Not Going Away'Top U.S. intelligence officials have yet to confirm Islamic State's claim of responsibility for the deadly Manchester attack, but they warn the incident is a reminder that the threat of terrorism around the world is real and "is not going away." "We have not verified yet the connection," Director of National Intelligence Dan Coats told lawmakers Tuesday during a hearing on worldwide threats, noting "they [Islamic State] claim responsibility for virtually every attack." He stressed that the attack "again reminds us that this threat is real, it is not going away, it needs significant attention to do everything we can to protect our people from these kinds of attacks." The so-called Islamic State claimed "one of the soldiers of the caliphate" placed an explosive device "within the gathering of the crusaders" in Manchester. While the claim has yet to be confirmed by Western intelligence officials, some former officials believe the links between the Manchester attack and IS will pan out. "The speed with which they took credit lends credibility to their being responsible for it," Robert Gates, a former CIA director and former U.S. defense secretary, told an audience at the Foundation for Defense of Democracies in Washington on Tuesday. Gates said whether the attack was planned by IS or merely inspired by the terror group is "immaterial as long as they [IS] were a spark."  New wave of terror He also expressed concern the violent strike may be the start of a new wave of terror by IS and its followers. "You will see ISIS become more active and more aggressive in a variety of places," Gates warned, using an acronym for Islamic State. "Sadly, Manchester may be a harbinger of more such activities in the West." U.S. intelligence officials have warned for years that the fall of Islamic State's self-declared caliphate in Iraq and Syria could trigger a terrorist diaspora, with former IS fighters fleeing to other ungoverned spaces or even to their countries of origin to continue the fight. And even with the Islamic State's Iraqi capital of Mosul on the verge of falling and the group's Syrian capital of Raqqa expected to fall by the end of the year, U.S. officials expect the group to remain a lethal presence, both in the Middle East and in the West. "ISIS has prepared for the loss of key territory in Iraq and Syria by publicly de-emphasizing the importance of territorial holdings to the caliphate's survivability," Defense Intelligence Agency Director Lieutenant General Vincent Stewart warned lawmakers in prepared testimony. "We anticipate that ISIS will attempt to maintain its narrative as the true defender of Muslims around the globe and will continue to plan and execute attacks against the United States and the West," Stewart added. There are also indications that the IS leadership, despite having been hit hard by U.S. airstrikes, remains intact, with key officials relocating to the group's remaining safe havens along the Euphrates River and to al-Qaim in Iraq's Anbar province. From there, IS leaders "will continue to plan and execute attacks against the United States and the West," the DIA warned. Evolution of IS Intelligence officials also point out that for more than a year, IS leaders have actively downplayed the need for followers to come to its self-declared caliphate. Instead, IS adherents have been encouraged to stay where they are and fight there.  There have also been lingering concerns that IS has managed to establish numerous sleeper cells across Europe, some using a combination of former foreign fighters and would-be homegrown terrorists, preparing to carry out attacks. "ISIS remains a dangerous enemy," a National Security Council official told VOA last month on condition of anonymity, given the ongoing review of the U.S. anti-IS strategy. Included in that review, the official said, has been an effort "to understand and prepare for the evolution the group might undertake." But many argue defeating IS will require much more than simply hunting down members. "That young man who carried out that suicide attack [in Manchester] and the ISIS members who posted about that attack … they were taught by someone," Republican Representative Ed Royce of California said at a Washington forum Tuesday. "There's an infrastructure behind that teaching," Royce cautioned. "These are not ideas that people adopt without a fair amount of brainwashing."