WASHINTON - The CIA base attacked by a suicide bomber in Afghanistan was at the heart of a covert programme overseeing strikes by the agencys drones along the Pak-Afghan border, The Washington Post reported Friday. Citing officials familiar with the installation, the newspaper said the assailant, wearing an explosives belt under his clothes, apparently was allowed to enter the base after offering to become an informant. The CIA declined to comment on the circumstances behind the incident, and it was unclear whether the bomber chose the base because of its role in supporting CIA airstrikes against top Al-Qaeda and Taliban leaders in the region. The blast early Wednesday evening in Khost killed seven CIA officers and contractors, including the base chief, and seriously wounded six others in what intelligence officials described as a devastating blow to one of the agencys key intelligence hubs for counterterrorism operations. It was the deadliest single day for the agency since eight CIA officers were killed in the 1983 bombing of the US Embassy in Beirut. Two of the people killed were contractors with the private security firm Xe, once known as Blackwater, an ex-intelligence officer told CNN. The CIA continued drone strikes Thursday - a missile attack by a Predator drone in Pakistans North Waziristan region, across the border from Khost. US intelligence officials vowed that the Wednesday attack would only increase the agencys resolve. This attack will be avenged through successful, aggressive counterterrorism operations, an unnamed official was quoted as saying. The CIA deaths were formally acknowledged by the agency in a statement to employees Thursday by Director Leon Panetta, who said the heavy toll was a reminder of the real danger that confronts intelligence officers on the fronts of the two wars. CIA operatives in Afghanistan volunteer for the posting and spend a year or more on assignment. Many of the slain including the base chief, a mother of three young children were seasoned hands in the agencys counterterrorism operations, The Post said. Those who fell yesterday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism, Panetta said in his message to employees. We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives a safer America. Panetta said military doctors and nurses had saved the lives of gravely wounded officers, and he announced that flags at CIA headquarters in Langley would be flown at half-staff to honour the dead. The CIA declined to identify the victims, according to The Post. Director of National Intelligence Dennis Blair did not publicly comment on the deaths, but a spokesman said he sent an internal, classified message expressing his condolences. President Barack Obama posted a letter to CIA employees honouring those killed, whom he called part of a long line of patriots who have made great sacrifices for their fellow citizens, and for our way of life. US personnel at the site of the attack, Forward Operating Base Chapman, are heavily involved in the selection of al-Qaeda and Taliban targets for drone aircraft strikes, the newspaper said, citing two former intelligence officials who have visited the facility. The drones themselves are flown from separate bases in Afghanistan and Pakistan. Because of its location near a hotbed of insurgent activity, the base is also a centre for recruiting and debriefing informants, the officials said, and it would not be unusual for local Afghans to be admitted to the facility for questioning. Theres still a lot to be learned about what happened. All the facts are not in, CIA spokesman George Little said. The key lesson is that counterterrorism work is dangerous. A Taliban spokesman asserted responsibility Thursday for the bombing and said the bomber was an Afghan National Army officer who had joined insurgents in attacking the US. That description, The Post said, could not be confirmed with US military officials. But a US military official in Afghanistan, speaking on the condition of anonymity, said Afghan forces are posted at the base. Forward operating bases in Afghanistan depend on locals for security, it said. But insurgents have frequently infiltrated the ranks of Afghan security forces as well as private firms hired to guard US facilities or to perform more menial tasks. CIA officials on Thursday would not discuss what guard service they had at the base. Forward Operating Base Chapman is a former Afghan army installation and was used jointly by American and Afghan security forces during their military campaign against the Taliban beginning in 2001, according to the newspaper. In recent years, the base added an intelligence-gathering function and had a housing compound for US intelligence officers. It was physically separate from the main US military base nearby, Forward Operating Base Salerno. Senior Afghan civilian officials in Khost said that they knew little about what went on at Chapman and that since Wednesdays attack, they have been unable to reach anyone inside by phone. Afghan interpreters working on the base at the time have since been incommunicado, and those who were on leave that day have not been allowed back inside, according to Khost residents and officials reached by phone. A spokesman for the Afghan National Army in Kabul denied that the Khost attack was carried out by a member of the army, but the possibility highlights growing concerns in Afghanistan and Pakistan about whether it is possible to sustain the loyalty and unity of their respective armies. The Afghan army, a crucial element in the new US strategy in Afghanistan, is young, untested and ethnically diverse. It is being asked to fight fellow Muslims from the dominant Afghan tribe in an unpopular war on behalf of American forces and policies that many Afghans deeply resent. This attack shows that the Taliban are getting good cooperation from the locals and that they have better intelligence than the Americans do, Talat Masood, a Pakistani security analyst and retired general in Islamabad, was quoted as saying. It also raises the issue that has haunted the Afghan National Army from the beginning whether or not it is possible to build a unified army that can overcome ethnic loyalties in support of broader American goals.