Sam Abel is an agent with American Intelligence.
Do not look for a better explanation of what agency or bureau or department of the American Intelligence community in specific that he works for because it is improbable you will find one. Certainly Abel gives no indication he knows for sure but then again, it is likely that Abel would not care one way or the other.
The time frame for the activities we have to follow of Abel is the days leading up to and into the Second World War. We first meet him just outside his small apartment in Brooklyn, carrying a couple of grocery bags, as he realizes someone who did not belong was in his humble residence. Abel knew the visitor was a stranger because the few people he knew would not have been so foolish as to intrude unannounced.
Which is why Abel figured the intruder was just a burglar; this misconception being put to rest when the man inside indicated he was there to have a chat. Moreover, the man was polite enough to inquire by which name Abel would prefer to be addressed - if not Abel then perhaps Stein or Abelson. "Though Sam had done well in ditching his previous identities, this man seemed to have him pegged."
The intruder would be thought of by Abel from that moment as 'the burned man' due to his appearance; scarring on his cheek, a patch of hair missing near the temple, and the skin around there red and misshapen. "He had suffered a severe burn and lived to tell the tale". Abel would learn much, much later that the burned man was named Hank Brandt but by then thinking of him just as the burned man seemed normal.
Brandt was bringing to Abel an offer of a job and with it, a chance to make his identity legitimate and not just an alias to use for a while before moving to a new city or even country and becoming someone else for a time.
When Abel inquires as to who the man's employer is that can do such a thing, the burned man says simply, "That's irrelevant" and apparently it was because neither seemed concerned about it enough to bring it up again. He knew then that he was going to work for the burned man, making use of his "very special skills" - namely, espionage.
Abel is described as being "a handsome man - tall, fit, and his hypnotizing blue eyes made him quite popular with the single women in the neighborhood. A good head of thick black hair didn't hurt. There was a natural wave in his scalp. He was muscular, but not bulky. There was something callous about the eyes". Some people who would make an enemy of Abel would learn that it was not just the eyes that could be callous.
- About a man Abel was investigating, "Moreno was a handsome man, and well-liked by his ... contacts who knew no better".