Washington (CNN) -- In the wake of the suicide bombing at Forward Operating Base Chapman, new security guidance has gone out to U.S. bases across Afghanistan, U.S. military officials said.
The December 30 suicide attack killed seven CIA officers and contractors, along with a Jordanian military officer who was the attacker's handler. Former CIA official Robert Richer called it the greatest loss of life for the agency since the 1983 bombing of the U.S. Embassy in Beirut, Lebanon, which killed eight agents.
A U.S. intelligence official, speaking on condition of anonymity, told CNN on Wednesday the attack was believed to have come on orders from "the highest levels" of al Qaeda.
Bruce Reidel, a former CIA officer who has advised President Obama on al Qaeda, said the attack showed the capacity of terrorists to strike at U.S. targets remained "very significant."
"It's a very, very sophisticated operation," Reidel said. "It must have taken a long time to plan and to set up."
In response, U.S. military officials said they were changing procedures.
"Suffice it to say that after any serious incident we evaluate the circumstances, think through the threat implications given our current practices and disseminate guidance to the field if adjustments are necessary," a senior military official said. The official would not describe the new security procedures because "we don't broadcast the specific changes we've made to security postures or our intelligence procedures."
A second U.S. military official said the guidance is meant to adjust procedures as quickly as possible on a large scale. "We don't want just the military intelligence guys looking out for specific things. We need MPs and ground forces to be aware," the official said.
The official said the security guidance is meant to be disseminated to as wide a group as possible, while more specific guidance on intelligence operations is kept to a much more limited, restricted distribution.
The officials had not seen specific guidance on handling sources, but both noted only one week has passed since the incident, and the investigation may not have progressed far enough to produce formal recommendations in that area. One official said the intelligence officers on the ground now know enough details to "self-correct" procedures.
According to sources, the Jordanian suicide bomber was Humam Khalil Abu-Mulal al-Balawi, a one-time militant who then appeared to be helping the United States. Al-Balawi was offering information on the possible whereabouts of Ayman al Zawahiri, the deputy to Osama bin Laden, the sources said, and was picked up outside the base and driven into it without being checked.
The sources said al-Balawi detonated the bomb shortly after arriving.
The attack shed light on the role of Jordan as a previously covert partner in the U.S. hunt for bin Laden and Zawahiri.
"The Jordanian intelligence service is the best intelligence service in the Middle East and South Asia, bar none," Reidel said. "They are far more effective in working against jihadist groups like al Qaeda in Iraq like the al Qaeda core in Pakistan than any other intelligence service."
CNN's Barbara Starr and Pam Benson contributed to this report.