WASHINGTON (AP) -- Law enforcement and intelligence agencies remain extremely concerned that violent militants will eventually flow out of Syria and Iraq and into other countries in hopes of committing attacks, FBI Director James Comey told Congress on Tuesday.
The number of Americans traveling to Syria to fight alongside the Islamic State group has slowed to a trickle in the last year, but as the so-called caliphate becomes "crushed," many militants from Western nations who are already there will stream out of the region and create new security threats.
"There will be a terrorist diaspora sometime in the next two to five years like we've never seen before," Comey said in testimony before the Senate Homeland Security and Government Affairs Committee.
Comey was testifying alongside Homeland Security Secretary Jeh Johnson and Nicholas Rasmussen, director of the National Counterterrorism Center, at a hearing examining threats to national security 15 years after the 9/11 attacks.
The hearing took place just over a week after bombings in New York and New Jersey and a separate stabbing attack at a Minnesota mall.
Rasmussen said that in addition to the Islamic State militants, U.S. government officials are concerned about the capabilities and ambitions of al-Qaida and its affiliates.
Johnson said terrorist threats have evolved, moving from terrorist-directed attacks "to a world that also includes the threat of terrorist-inspired attacks" in which individuals who live in the U.S. are "self-radicalized" to attack their own country.
Johnson says that by their nature, terrorist-inspired attacks and terrorist-enabled attacks are difficult to detect by intelligence and law enforcement communities, can occur with little or no notice and in general make for a more complex homeland security challenge.
The panel's chairman, Sen. Ron Johnson, R-Wis., said the threat of "militant Islamic terrorist attacks to the United States remains significant," citing the Sept. 17 attacks in the New York region and Minnesota, as well as deadly attacks in San Bernardino, California, and Orlando, Florida.
"In all, Islamic extremist terrorist have killed 63 people on U.S. soil since our committee last held its annual hearing to consider threats to the homeland," the chairman said in a prepared statement.
Two years after President Barack Obama stated a goal of defeating the Islamic State group, also known as ISIS, "we have made little progress," said the senator, who is not related to the Homeland Security chief.
Ahmad Khan Rahami, the main suspect in the New York bombing, faces federal terrorism charges after a shootout with police.
Prosecutors say Rahami, 28, planned the explosions for months as he bought components for his bombs online and set off a backyard blast. They say he wrote a journal that praised Osama bin Laden and other Muslim extremists, fumed about what he saw as the U.S. government's killing of Muslim holy warriors and declared "death to your oppression."
While Republican presidential nominee Donald Trump has suggested a temporary ban on Muslim immigrants, Jeh Johnson said the U.S. should focus on "building bridges to diverse communities" to defend the homeland.
Lawmakers also may focus on police shootings in Tulsa, Oklahoma, and Charlotte, North Carolina, as well as Hillary Clinton's use of a private email server while she was secretary of state.
Republicans have assailed Comey's decision not to prosecute Clinton, now the Democratic nominee for president. Several have demanded the Justice Department investigate whether Clinton lied during testimony last year on the deadly 2012 attacks in Benghazi, Libya. Democrats call the claims partisan and intended to hurt Clinton's candidacy.