The homicide bombers who struck Baghdad this week used thousands of pounds of plastic explosives and diversionary tactics that are hallmarks of Al Qaeda (search), a senior U.S. counterterrorism official said Thursday.

The way the attacks were carried out has made American investigators believe they could be seeing a change in tactics by those attempting to drive U.S. forces out of Iraq (search), the official told The Associated Press in an interview.

"We're looking at that as the start of something that may be forewarning of future events," said the official, who spoke on condition of anonymity.

The New York Times reported on its Web site Thursday night that U.S. officials believe Saddam Hussein (search) himself may be playing a significant role in coordinating and directing attacks by his loyalists.

The Times, citing unidentified senior officials, said recent intelligence reports have portrayed Saddam as a catalyst or even a leader in the armed opposition, probably from a base of operations near his hometown of Tikrit (search). The reports have not been corroborated, one official told the newspaper.

In a speech Thursday night, Defense Secretary Donald H. Rumsfeld, said attacks are practically inevitable as long as Saddam remains on the loose.

"The fact that he's alive is unhelpful," Rumsfeld told the conservative think tank Empower America. "We do need to catch him and I think we will."

Before Monday, bombers in Iraq had largely used relatively crude devices cobbled together from old Soviet-era munitions. "People went out and got anything they could find," the official said.

But each of the vehicles used in the latest attacks, which targeted the International Red Cross headquarters and four Iraqi police stations, was packed with 1,000 pounds of plastic explosives, the official said. A fifth bomber was prevented by police from detonating his device and was taken into custody.

The four successful bombings, timed within 45 minutes of each other on the first day of the Muslim holy month of Ramadan, killed 40 people in the deadliest single attack since U.S. forces took over the city.

In at least three of the attacks, witnesses said a vehicle was used to distract or confuse perimeter guards to allow the bomber's vehicle to slip through. Al Qaeda attackers used lead vehicles to overcome security and get explosives-laden vehicles into guarded compounds housing Westerners in the May 12 bombings in Riyadh, Saudi Arabia. Those bombings killed 35 people, including nine attackers.

But other, outside experts cautioned that Al Qaeda tactics could easily be mimicked by other terrorist groups.

"The problem is, these are the hallmarks of a lot of people," said Anthony Cordesman, a national security expert at the Center for Strategic and International Studies. "The weapons are there, and people are gaining the experience."

The issue of who is behind the escalating attacks in Iraq is proving to be one of the most difficult and puzzling for American officials.

President Bush said in a news conference this week that both Saddam loyalists and foreign fighters might be behind the attacks.

At the Pentagon, Rumsfeld told reporters Thursday it was not yet clear whether this week's spiraling violence is part of a larger new offensive by insurgents or just a short-term surge.

"It's hard to put it in perspective while it's still going on," Rumsfeld said.

Plastic explosives have previously been found among the munitions believed left over from Saddam's rule. Coalition forces have seized large caches of weapons, including blocks of plastic explosives last week in a government building in Saddam's hometown of Tikrit. A raid last month in Tikrit uncovered 1,000 pounds of plastic explosives.

The FBI has traced the particular explosives used in Monday's attacks to a manufacturer outside of Iraq, the counterterrorism official said.

The official said American investigators believe Iraq is becoming a magnet and proving ground for Islamic militants around the world. The United States has evidence that hundreds of militants have entered Iraq to battle Americans, much as they did a quarter-century ago to fight the Soviets in Afghanistan.

Rumsfeld said as many as 400 foreign fighters have been captured in Iraq, most of them from Syria and Lebanon.

The FBI says it is making progress toward solving some of the Iraq attacks, with assistance from the CIA and Defense Department. But the investigations are being hampered by a lack of intelligence information in Iraq and by the sheer level of violence, which requires agents to be accompanied by U.S. soldiers when they leave their protected compound at Baghdad airport.

If suspects are identified, U.S. officials will probably monitor them to try to identify their associates and contacts and to prevent future bombings. Evidence also is being gathered that could be used in a criminal trial, possibly in Iraq and possibly in the United States in cases where Americans were killed.