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(NBC) - Top U.S. security officials are now calling the killings at the U.S. consulate in Benghazi, Libya last week a terrorist attack.

What remains under debate is whether that attack was planned or spontaneous.

Sitting before the Senate Homeland Security Committee Wednesday three of the top U.S. security officials called the deadly assault on the consulate, where four Americans died, evidence the U.S. continues to be vulnerable to terrorist attacks.

"They died as a result of a terrorist attack," National Counterterrorism Center Director Matt Olsen said bluntly.

But much else remains unanswered or under dispute.

Was Al Qaeda involved?

"We are looking at indications individuals may have had connections to al Qaeda," Olsen testified.

Was the attack planned like the Libyan government insists, or was it spontaneous as U.S. intelligence officials believe?

"I just don't think people come to protests equipped with RPGs and other heavy weapons," Senator Susan Collins noted.

As investigators sort out the facts the families of those men killed are laying their loved ones to rest.

Collins said the deaths could have been prevented had the consulate been better protected and asked why the U.S. relied on a limited number of Libyan forces for protection instead of U.S. Marines.

"I'm just stunned and appalled their wasn't better security," Collins said.

Secretary of Homeland Security Janet Napolitano says the real threats related to the video that sparked the consulate attack have remained overseas.

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