Aleks Thornton's identity document found in TBOGT's files shows he is an agent, more specifically, the Director, of the Central Intelligence Agency.
His file mentions the Algonquin borough of Liberty City, most likely the place where he lives/operates in; the file also mentions "Berkley House", which is probably the name of his residence place.
His phone number is 0001-4742-6002-2006, while his fax number is 0001-4742-6002-2010.
OFFICIAL C.I.A. AGENT
DCI Aleks Thornton
Tel: 0001 4742 6002 2006
Fax: 0001 4742 6002 2010
He is the first known CIA agent in the HD Universe of the series. While Grand Theft Auto V features an in-game version known as IAA, it is most likely this group was created after TBOGT was released, in 2009.
DCI stands for "Director of Central Intelligence", although the position was replaced by "Director of the Central Intelligence Agency" and "Director of National Intelligence" in 2005.