Kabul , Afghanistan -LRB- CNN -RRB- -- An Afghan employee of the U.S. Embassy in Afghanistan who opened fire at a CIA annex there worked as part of a local guard force , a U.S. official said Monday . Embassy spokesman Gavin Sundwall said the lone gunman killed a U.S. citizen and wounded another in the shootings Sunday . The wounded American was evacuated to a military hospital with injuries that were not life-threatening . Security personnel used a flash-bang round to stun the shooter and then killed him , according to another U.S. official who earlier provided details of the incident . The gunman shot indiscriminately , and he is not believed to have targeted the victims , the second U.S. official said . One or two civilians suffered minor wounds as people went running for cover , the official added . The American killed was a CIA employee , according to a third U.S. official . The official said the gunman fired toward the building but never entered it . All the sources spoke on condition of not being identified . The FBI is investigating the attack , along with other U.S. agencies and local authorities , a U.S. official said . Another official said the FBI is the lead agency in the investigation . CIA and diplomatic security officials also are involved . The first U.S. official said the shooter `` is believed to have worked as part of the local guard force '' and added that no motive has been established . Investigators are looking into whether the shooter was a disgruntled employee or whether he was inspired by militants . All Afghan employees go through a background security check and must pass through a metal detector each time they enter the Embassy , the second official said . `` We have no information as to whether the individual was authorized to carry a weapon or if he seized a weapon , '' the official said . Afghan troops guard the outer perimeter of the Embassy , while embassy contractors and members of the U.S. military guard the inside . The CIA building is several hundred yards from the Embassy compound and has its own entrance and security , according to another U.S. official . Embassy personnel usually travel in armored cars for meetings in the CIA facility . `` It 's not possible to stop everything from happening . You can only limit it , make it highly unlikely , '' said the official , who also spoke on condition of anonymity . `` The job of soldiers and diplomats and spies is to be amongst the people , getting work done . You have to have some level of trust to get anything done . '' `` There is a calculated risk '' for U.S. personnel working in countries like Afghanistan , the official said . The annex area is known as a place where Embassy personnel both live and work , some in intelligence operations . A CIA spokeswoman declined to comment on the attack . Afghanistan has been the site of several high-profile attacks of late , including strikes at the NATO headquarters and the U.S. Embassy in Kabul and the assassination of former Afghan President Burhanuddin Rabbani . Rabbani had been leading efforts for reconciliation talks . The latest shooting is not believed to be related to any other recent attacks in Kabul , U.S. officials said . In December 2009 , a Jordanian informant detonated a suicide bomb at a CIA base in Khost , Afghanistan , killing seven CIA employees and two others . A CIA review of the incident found that some people within the CIA and the Jordanian intelligence service were skeptical about the attacker 's reliability , but those concerns had not been passed on to officers on the base . CNN 's Barbara Starr , Nick Paton Walsh , Carol Cratty and Pam Benson contributed to this report .