Raymond Allen Davis, 36, was working as a CIA contractor for the US consulate in Lahore, the officials say.
Mr Davis is being held in jail pending a decision on his diplomatic status.
The former soldier shot two men he says were trying to rob him. The incident has soured US-Pakistan relations.
Correspondents say the Pakistan government is under tremendous domestic pressure to put Mr Davis on trial despite US assertions that he has diplomatic immunity. Mr Davis is charged with two counts - murder and possession of illegal weapons.
Hundreds of Pakistanis have taken part in street demonstrations.'Protective officer'
The US State Department on Monday repeated the administration's stance that Mr Davis was an accredited member of the technical and administrative staff of the US Embassy in Islamabad.
They said the Pakistani government had been informed of his status in January 2010 and that Pakistan is violating its international obligations by continuing to hold him.
Mr Davis was working as a "protective officer," Reuters news agency quoted sources as saying.
US media outlets, including the New York Times, the Washington Post and the Associated Press, said they had held off publicising details about Mr Davis' employment status while the US tried to improve his security situation.
But on Sunday, the UK's Guardian newspaper disclosed Mr Davis' relationship with the CIA.
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