Counterterrorism officials are taking seriously a claim by a little-known group calling itself "The Secret Organization of Al Qaeda in Europe" that it staged the deadly attacks on the London transit system.

Among theories investigators are pursuing is whether the group may be linked to Iraq's terror chief, Abu Musab al-Zarqawi (search).

Current and former government officials agree Thursday morning's attacks were trademark Al Qaeda: near-simultaneous explosions, using improvised devices, aimed at Westerners.

By Thursday evening, a senior U.S. counterterrorism official acknowledged that the Internet posting by Al Qaeda in Europe was considered a "potentially very credible" claim, in part because the message appeared soon after the attacks and didn't appeared rushed.

But no one was certain, and one defense official said it was too early to say.

"Even though they have come out and staked a claim, I think everyone is still running all the traps," the official said.

All officials spoke on condition of anonymity because investigations were ongoing.

More than a half-dozen current and former intelligence officials interviewed Thursday conceded that Al Qaeda in Europe was a new name to them. But others said it could be an existing group, seeking notoriety with a brutal attack and new title.

Officials couldn't say whether the organization behind Thursday's posting — or the attacks — was considered part of the Al Qaeda core answering to Usama bin Laden, an affiliate group receiving support from the main organization, or just an ideological cousin.

Clues also suggested that the rail and bus bombings might be linked to al-Zarqawi, who pledged allegiance to bin Laden last year after he successfully coordinated or executed dozens of grisly terror attacks in Iraq. Jordanian-born al-Zarqawi has indicated a desire to reach beyond Iraq and attack Western targets, and intelligence reports indicate bin Laden has encouraged him.

Vince Cannistraro, a former CIA counterterrorism chief, said links to al-Zarqawi were certainly possible. Britain's domestic intelligence agency has known about operatives related to him inside the country for some time, including a cell that was disrupted in early 2003 making ricin in a London apartment.

But, Cannistraro said, sorting out the relationships is increasingly difficult because Al Qaeda no longer has a topdown structure. "They are depending on affiliates and groups around the world. They encourage them, they send them money, but it is not like they can plan from Kandahar" in Afghanistan.

That base, he and others often remind, is gone.

After the fall of the Taliban in 2001, senior intelligence officials including CIA Director Porter Goss have warned that groups ideologically driven by Al Qaeda have formed a broader movement able to carry out attacks independently.

Organizations with names similar to Al Qaeda in Europe emerge only to sometimes disband a short time later. Al-Zarqawi is thought to have operated under a number of different names in Iraq, most recently using Al Qaeda Between Two Rivers or Al Qaeda in Iraq. One senior counterterrorism official said the name's formula was akin to Al Qaeda in Europe, a potential hint they may be linked.

John Rollins, a former senior intelligence official at the Homeland Security Department (search), said Al Qaeda's transition to ideology that doesn't necessarily answer to a central organization makes its followers more dangerous and hard to track.

"Anyone can say, 'We are Al Qaeda in Montana or Canada,"' said Rollins, now with the Congressional Research Service. "Al Qaeda is no longer your daddy's Ford. It is a new animal."