June 13, 2007 — -- In an exclusive interview with ABC's Charles Gibson, Gen. David Petraeus, the top U.S. commander in Iraq, called today's attack on a holy Shiite shrine near Baghdad a "serious blow" to the military effort in the region and the work of al Qaeda.
"But frankly, it is our hope that this can galvanize the Iraqi leaders to unite against this form of extremism," Petraeus said from Baghdad.
"Iraqi leaders have done what I think everyone would've hoped they would do in the wake of such a tragedy, such a horrific attack, and that is to condemn it universally, and again, call for calm in the wake of it," he said.
This was the second attack on the golden-domed al-Askariya mosque in Samarra, 65 miles north of Baghdad and the burial place of two imams that had been guarded by dozens of Iraqi commandos since the first attack.
U.S. military officials told ABC News the mosque's minarets appear to have been toppled by explosives placed inside the structure, which suggests a serious security lapse.
Watch the interview tonight on "World News With Charles Gibson"
When the mosque was hit in 2006, the incident triggered widespread sectarian violence. Petraeus said he and his forces are already working to avoid reprisals.
"When we met earlier today a number of provisions were immediately ordered by Prime Minister Maliki. Our forces have helped Iraqi forces put those in place. We're helping to move reinforcements to Samarra from the Iraqi national police," he said.
Maliki has also ordered that 15 guards from the area be detained for questioning.
While there will be a full investigation to determine who planned the attack, Petraeus said it had the "hallmarks" of an al Qaeda effort.
"We do not have much doubt that this is al Qaeda Iraq, or at the least an element that is very connected to them," he said.
"This is very characteristic of their mode of operation, not of the mode of operation by the extremists connected with the Sadr militia or some of the other groups. It also took some degree of expertise in explosives," he added.
And he said that although after the mosque was hit he initially had a "terrible sinking feeling," he believes there is reason to be optimistic in the fight against al Qaeda in Iraq.
"They are under a fair amount of pressure. I think they know that we are going to contest some of the areas in which they have had sanctuaries in the past," Petraeus said.
"There is even some hope, perhaps, that al Qaeda may have overplayed its hand, as it did in Anbar Province, as it has in some neighborhoods in Baghdad, and in some other areas where, as you know, Sunni Arabs have rejected al Qaeda and have actually sought to join coalition forces and Iraqi governmental institutions to fight against it."
ABC News' John Hendren contributed to this report