They would not say whether Iraqi guards or personnel from other countries were also present.
The small staff, which included some armed Iraqi guards, was taken hostage.
Last week, Iraqi guards kept weapons inspectors from entering three sites.
A suicide bomber and an Iraqi guard died in a second attack at the headquarters this week.
At one point, Iraqi guards appeard and threatened to make them stop.
An attacker and an Iraqi guard were killed in a firefight.
For the rest, a trained Iraqi guard can be hired for $25 a day or less.
One Iraqi guard and one attacker were killed, officials said.
He said that he did not shoot first, but was fired at by Iraqi guards in the building.
(Inside the building, there is now a company that supplies food to the Iraqi national guard.)