Production design for the film

This was one of the films that really got me interested in the architecture of Chicago - something that I knew nothing about until I actually looked into Chicago in detail trying to find one location featured in this film:- Richard Thorn's office.

I knew that the exterior and lobby are Chicago City Hall (which is truly beautiful - as I posted this information on the locations board), but did not know whether the offices that feature as the corridor that William Holden and Lew Ayres walk down, together with the dining room where Holden and Robert Foxworth have breakfast and Holden's magnificent office (which seems to look towards the distinctive red CNA Building in Chicago were one/several real location(s) or just an impressive and convincing stage set.

I'd really like to know where this/these location(s) were found, together with some others in Chicago (or possibly Los Angeles), such as the roads in Chicago and Los Angeles where Elizabeth Shepherd's death sequence was filmed, as Ii really do think that this is an underrated film, partly because it lends so much deference and obvious affection to "The Omen", the film that spawned it.