Baghdad ? The bomb that devastated the U.N. complex in Baghdad was a potent blend of Soviet-era artillery shells, mortar rounds and grenades packed around a powerful centerpiece — a 500-pound bomb meant to be dropped from an aircraft, the FBI said Wednesday.

“This was not a homemade bomb,” said Thomas Fuentes, the FBI official heading the investigation. “We’re talking about highly powerful, military-grade munitions.”

Investigators are considering a wide array of potential attackers, including Saddam Hussein loyalists, foreign and domestic terrorist groups and religious extremists — or some combination of these and others enraged at the U.S.-led occupation.

“There’s so much of this munitions in the country, it could have been anyone,” said Fuentes, who called the bomb’s contents “a munitions lasagna.”

The use of weaponry once in the largely Soviet-equipped Iraqi arsenal strongly suggests a connection to Saddam loyalists. The munitions were standard issue and would have been relatively easy to obtain for any Iraqi close to the nation’s former security apparatus. And many former military men in Iraq were well-trained in explosives and sabotage.

The FBI said it was too early to say whether the bombing was a purely Iraqi operation or involved foreign collaborators. The sheer size of the bomb suggested at least an operation involving several people.

And the audacity and precision of Tuesday’s attack suggest a foreign hand, according to several Iraqis.

“There is a feeling, based on accumulated data from the past, that it is the remnants of Saddam’s regime and their ‘friends’ ” who staged the attack, Ahmad Chalabi, a member of the fledgling Iraqi Governing Council, said at a news conference Wednesday. Chalabi did not offer evidence of his claim.

The proliferation of munitions in Iraq underscores a deep irony: While no one has yet found the weapons of mass destruction that were the catalyst for the war to topple Saddam’s regime, the easy availability of high-powered explosives provides anti-U.S. militants with an almost limitless supply of conventional weapons and explosive material to wreak considerable havoc and destruction.

Foreign assistance?

Tuesday’s massive blast — the second in Baghdad in less than two weeks — added to the debate here about the extent to which foreign fighters have joined the campaign against U.S.-led forces.

L. Paul Bremer III, the U.S. administrator in Iraq, said earlier this month that several hundred operatives from the extremist group Ansar al Islam — who fled their bases in northern Iraq during the war — have slipped back into the country since May 1 in addition to radical Iranians and suspected members of the al-Qaida network. Some have used passports from Sudan, Yemen and Syria, according to Bremer.

The U.N. bombing was “of a different scale than the ones we’ve seen here before,” Bremer said after Tuesday’s attack.

“It does not mean … that we can exclude the possibility that the Fedayeen Saddam or some of the old Saddam guys did it. They had very substantial explosives capabilities in parts of their intelligence services, and it’s not impossible that it was done by them.”

The degree to which foreign and home-grown groups cooperate with each other remains a matter of debate. An emerging theory is that well-armed former Baath Party militants may now be teaming up with ruthless extremists from outside the country who see occupied Iraq as the new battleground against the West.

Suicide mission likely

Body parts discovered amid the truck wreckage point to a suicide mission in Tuesday’s attack, the FBI said, but officials are awaiting forensic examination of samples to be sent to the United States to be certain.

Another possibility is that the lethal package went off somewhat prematurely, not giving the driver or others in the truck time to escape.

U.S. intelligence officials, who are working with the FBI, said they have yet to determine who was behind Tuesday’s blast.

“Right now, there’s nothing that would point one way or the other,” one U.S. official said. “And no one we’ve seen has stepped forward.”

The official said a car-bomb attack that killed 17 people earlier this month outside the Jordanian Embassy in Baghdad involved some sort of dynamite-like explosives, not the aerial bomb, hand grenades and other military munitions used in the truck bomb at the U.N. headquarters.

“It’s too early to tell whether or not they’re related,” he said.