I was on the phone with mr. h when it happened; he works on 17th between 5th & 6th. In the middle of our conversation he said "What the #@$% was that?", I said "What was what?", and he said there had just been a loud explosion that rattled all the windows in the building. A minute later I could hear the sirens over the phone.
The thing about the Apex Tech building, and many buildings downtown for that matter, is that the facade is cast-iron (they did a lot of that in the early 1900's). In other words a large part of the interior could have collapsed and the outside would still be standing.
Thanks for first-hand report, pal. Glad your friend is okay.