The threat matrix that U.S. intelligence and law enforcement closely monitor to guard against terror attacks has ticked up in recent weeks as groups such as al Qaeda eagerly try to regroup from a series of leadership losses, including the killing of Osama bin Laden, federal officials say.

Officials caution that there is no specific, credible threat of an impending attack as Americans head into the Fourth of July holiday, but their growing caution is evidenced by a series of recent bulletins they’ve sent local law enforcement.

One bulletin last week disclosed that evidence found in bin Laden’s possession in May, when he was killed by U.S. special forces in Pakistan, indicated an interest in attacking Americans at major holiday gatherings, including Independence Day celebrations.

Another warned of Internet chatter that an al Qaeda affiliate had identified more than 40 prominent Americans the terrorist group wanted killed.

“There is a higher threat environment,” says Frances Fragos Townsend, a national security expert who advised George W. Bush on homeland security issues.

In past years, some holiday weekends have passed without incident after similar law enforcement warnings. But the increased vigilance comes as the Obama administration overhauled its anti-terror strategy earlier this week—after fresh signs of America’s vulnerabilities surfaced in New York, when a Nigerian man slipped past airport security with a fake ID and boarded a flight.

One development driving the concern of U.S. officials is the ascension of longtime Egyptian extremist Ayman al-Zawahiri as the new leader of al Qaeda.

Long the No. 2 to bin Laden, al-Zawahiri is an operational expert who lacks bin Laden’s magnetism and charisma, and U.S. officials suspect he may seek to win favor and loyalty among his lukewarm troops by trying to pull off a big attack soon against a Western target.

“We knew that in the aftermath of the Osama bin Laden killing, there was likely to be retaliatory attacks,” Townsend explained. “Al-Zawahiri was saying that he was planning something. The Pakistani Taliban said they were going to take revenge. All that suggests that we’re facing a higher threat.”

Clark Ervin, the first inspector general of the Homeland Security Department, says another reason al Qaeda and its followers may be striving to strike soon is that they are feeling less relevant after the Arab uprisings this spring used street protests, not terrorism, to push for increased freedoms from oppressive regimes.

"There are a variety of reasons why the threat of a terror attack is likely increasing—the death of bin Laden and the need for al Qaeda to avenge it and prove that they remain relevant, especially amid the ‘Arab Spring’ that runs counter to their narrative that the only way to topple the Mideast's autocrats is through violence, and that what should follow them is an Islamic theocracy rather than a democracy,” explained Ervin, now a security expert at the Aspen Institute.

Other reasons, Ervin said, are “Zawahiri's particular need to prove that, contrary to the speculation, he can lead al Qaeda effectively—and the approach of the 10th anniversary of 9/11, which would be a natural time to replicate that attack or come as close to doing so as possible.”

The chatter suggesting a retaliatory strike by al Qaeda and its friends among the Taliban also has ticked up on jihadist websites.

For instance, the FBI sent a bulletin to law enforcement June 8 noting that extremists had posted online a “target specific” list earlier this month identifying more than 40 people, mostly prominent Americans in business, politics and media, who should be assassinated. The list was quickly compiled on websites after a video surfaced featuring a call for jihad by American-born al Qaeda spokesman Adam Gadahn, one of the terror group’s favorite propagandists.

“While the information posted was detailed, it appears aspirational in nature and it is unknown whether the threat will progress beyond discussion," the bulletin said.

Al Qaeda-inspired spinoff groups in such countries as Yemen, Somalia, Pakistan, and Afghanistan also are drawing more scrutiny as their terrorist chatter has picked up in recent weeks. The Taliban was among those who recently promised to step up attacks.

One such attack was carried out earlier this week when operatives for the Taliban and its allied Haqqani network stormed a hotel favored by Westerners in Kabul, Afghanistan, killing at least 12 people; the nine assailants also died in the attack.

While U.S. officials are convinced several terrorist groups are increasingly determined to strike, there are legitimate questions about their capabilities—especially for a large-scale operation—after a series of devastating losses of top leaders that goes far beyond bin Laden.

For instance, NATO forces on Wednesday reported killing Ismail Jan, a key operational leader of the Haqqani network involved in the Afghan hotel attack earlier this week.

On June 11, Somali intelligence forces allied with the United States killed one of America’s most wanted terrorists, Fazul Abdullah Mohammed, identified as the mastermind of the bombings of U.S. embassies in East Africa in 1998.

Similar strikes by CIA drones also have thinned out the ranks of al Qaeda leaders hiding in remote regions of Pakistan, as well as some operational leaders of al Qaeda in the Arabian Peninsula, a Yemen-based group linked to a failed bombing attack against a U.S. airliner in Detroit on Christmas Day 2009.

The Yemen group is of particular concern to U.S. officials, in part because increasing civil unrest inside that country and the departure of its president, a longtime U.S. ally in the war on terror, have given the al Qaeda affiliate new inspiration to try attacks, current and former officials say.

“I think we’ve seen an increased level of operations of al Qaeda, particularly al Qaeda in the Arabian Peninsula,” Townsend said. “I think they’re frustrated that they haven’t been able to launch a successful, large-scale attack in a while.”

The special forces raid that successfully targeted bin Laden turned up evidence of previously unknown terrorist threats, and demonstrated that he was far more than a man on the run. He was considering all sorts of potential terror attacks and American vulnerabilities, U.S. officials say.

One set of documents found in bin Laden's hideout show that, as recently as February 2010, al Qaeda was considering attacking celebrations on major U.S. holidays, including Independence Day, a recent warning from the FBI shows.

Bin Laden was killed by U.S. special forces during a raid in early May at his hideout in Abbottabad, Pakistan.

Partly with new tools and partly through luck—some attempted attacks like car and airliner bombings have fizzled—U.S. officials have been able to thwart any major new attacks on U.S. soil since the Sept. 11, 2001 attacks.

But officials are acutely aware that a growing number of terrorist groups, including domestic lone wolves and al Qaeda sympathizers, are determined to keep trying to strike—a point President Obama’s top official for counterterrorism, John O. Brennan, made this week in rolling out an update of the nation’s blueprint for fighting terrorism.

“Although this strategy focuses predominantly on the al-Qaeda linked and inspired threats, we also need to maintain careful scrutiny of a range of foreign and domestic groups and individuals assessed as posing potential terrorist threats, including those who operate and undertake activities in the United States in furtherance of their overseas agendas,” Brennan wrote in a 19-page memo made public Wednesday.

While the United States has significantly hardened its defenses over the last decade, fresh signs of vulnerabilities continue to surface.

On Thursday, officials acknowledged that a man of Nigerian descent managed to get past security and onto a flight at New York's John F. Kennedy International Airport with a fake ID and stolen boarding passes. The Transportation Security Agency was investigating how its safeguards were foiled.

And last month, the chief of America’s border patrol agency warned that with so much focus on violence on Mexico’s borders, U.S. officials remain concerned about terrorists passing into the United States from Canada.

“In terms of the terrorist threat, it's commonly accepted that the more significant threat, because of the population and because of certain relationships with Canada, [is] people who can enter Canada and then come across our bridges into the United States,” said Alan Bersin, head of U.S. Customs and Border Protection.

This post originally appeared at The Daily Beast.