WASHINGTON — After 13 years of struggle, Americans are showing a growing public fatigue and waning sense of urgency over terrorism, an attitude that threatens U.S. security, members of the panel that investigated the Sept. 11 attacks said in a report on Tuesday.

A decade after issuing the official account of the Sept. 11, 2001, attacks on New York and Washington, the former members of the bipartisan 9/11 Commission issued a new report warning that, while the world has changed dramatically, the threat from terrorism remains and has “entered a new and dangerous phase.”

“Many Americans think that the terrorist threat is waning – that, as a country, we can begin turning back to other concerns. They are wrong,” the report said. “The threat remains grave and the trend lines in many parts of the world are pointing in the wrong direction.”

“Al Qaeda-affiliated groups are gaining strength throughout the greater Middle East,” it added. “While the various al Qaeda spinoffs are primarily focused on regional conflicts, they hate the United States and will not forego opportunities to strike at the U.S. homeland.”

The panel, headed by former republican New Jersey Governor Thomas Kean and former democratic Representative Lee Hamilton, said foreign fighters returning to Europe from Syria and Iraq pose a “grave threat” to the United States and Western Europe.

Of the 10,000 foreign fighters that have traveled to Syria, more than 1,000 hold European passports, which in most cases would enable them to enter the United States without a visa, the panel said. More than 100 U.S. citizens also fought in Syria.

“When these battle-hardened, radicalized fighters return to their home countries, they will pose a serious terrorist threat to both the United States and Europe,” it said.

The group also noted that “The 9/11 Commission Report” issued a decade ago called for reducing the fragmented oversight that resulted in the Department of Homeland Security reporting to 88 committees and subcommittees of Congress.

The report said U.S. counterterrorism capabilities have improved significantly since the Sept. 11 attacks and people in government are tracking the evolving threat, “thinking one step ahead in order to prevent the next attack.”

It issued a series of recommendations, including calling on U.S. leaders to give the public detailed information about evolving threats, and asking Congress to update its 2001 authorization for the use of military force to improve the administration’s response to threats from Islamist militants.