President Bush says the beheading of an American civilian in Iraq shows the true nature of the terrorists who want to block the drive to freedom and democracy for the Iraqi people.

The president looked grim as he spoke of the death of Nick Berg. "Nicholas Berg was an innocent civilian who was in Iraq to help build a free Iraq. There is no justification for the brutal execution of Nicholas Berg," he said.

The beheading was shown on a videotape that appeared Tuesday on an Islamist Internet site. On the video, one of his captors read a statement, saying the murder was in retaliation for the abuse of Iraqi prisoners by U.S. soldiers.

President Bush said if the killers thought they could shake America's will, they were mistaken. "We will complete our mission. We will complete our task," he said.

The president said he sends his deepest sympathy to the Berg family. The Bush administration has vowed that the killers will be brought to justice.