WASHINGTON - President Barack Obama paid sombre tribute Friday to the seven CIA employees killed in one of the worst attacks in the history of the U.S. intelligence agency, calling them patriots who "served in the shadows and took pride in it" before paying the highest cost for freedom.

The White House released a transcript of Obama's remarks from a memorial service at CIA headquarters in a Washington suburb, which was closed to the media. More than 1,000 agency workers attended, as did family members of the employees who were killed during the suicide bombing attack in Afghanistan on Dec. 30.

"To their colleagues and all who served with them -- those here today, those still recovering, those watching around the world -- I say: Let their sacrifice be a summons," Obama said. "To carry on their work. To complete this mission. To win this war, and to keep our country safe."

The seven Central Intelligence Agency employees and a Jordanian intelligence officer were killed when a suicide bomber detonated his cache of explosives at Camp Chapman, a tightly secured CIA base in Khost province, a dangerous region southeast of the Afghan capital Kabul. The CIA had cultivated the bomber -- a Jordanian doctor identified as Humam Khalil Abu-Mulal al-Balawi -- in hopes of obtaining information about al-Qaida's second in command. Al-Balawi turned out to be a double-agent.

In a video broadcast after his death, the bomber said the attack was meant to avenge the death of the former Pakistani Taliban leader in a CIA missile strike.

Obama offered words of comfort to the parents and spouses of those killed. And to their children, he said: "I know that this must be so hard and confusing, but please always remember this. It wasn't always easy for your mom or dad to leave home. But they went to another country to defend our country."

The names of those killed were edited out of the transcript to preserve confidentiality. Obama said the work of the seven employees, like that of the CIA more broadly, is unknown to Americans but is recorded forever in the terrorist attacks that were thwarted and the lives that were saved.

Said Obama: "They served in secrecy, but today every American can see their legacy."