Storm: Roof Leaking

Hello,
Today we are having heavy rains here around San Antonio, TX and water started dripping from my living room ceiling. The location is about 24" from my chimney.
As soon as the lightning passed, I got up on the roof to inspect. I couldn't really see anything out of place regarding the shingles in the area, everything looked pretty okay. There was one suspect spot where I applied some roofing tar "just in case."
I did find however, that one of the bricks that makes up the top of the chimney was loose and came right out in my hand. It is on the side where the water is showing. This seems too coincidental not to be related, though I don't understand how it is constructed enough to know if there is a way for rain to go in through those cracks in the grout and end up soaking the sheetrock.
Wouldn't the water just end up in the chimney or firebox? I covered that brick with a board for now until I can get it repaired. I am waiting for more rain (it is expected) so I can see if the water continues to come in.
Thoughts?
ER
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it done right. Tom Work at your leisure!
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The actual brick "chimney" as it is, expands about six feet wide by 4 feet deep. As I now realize, the actual smoke stack that leads from the firebox in the living room up to the roof is only about 2x2. The bricks around it are for the "look" and leave some great empty space in-between. So now I understand how water could get behind those bricks and then into the house--as opposed to just getting the firebox wet. See, I thought I had a chimney like you would see in an older home--made of all brick and that was that.
I bought some mortar and repaired the brick--we'll see how it goes.
ER
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