Armstrong was swimming with her husband and a group of lifeguard friends Thursday when she decided to return to shore.
The group later noticed a large shark, approximately 11 feet long, swimming near them. Fearing for their safety, the group stayed close together and then swam back to the beach club.
But Christine Armstrong was nowhere to be found.
Police Inspector Jason Edmunds told CNN that a witness reported seeing a large shark attacking what appeared to be a swimmer 150 yards off-shore.
"The community is in shock, and the husband is taking it hard, as he has been with her since they were kids.. Cancer, heart attacks -- those are all things we fear but expect, but who gets taken by a shark?"
Edmunds noted that large shark sightings in the area are rare.
Swim Week Miami Beach 2014 [PHOTOS]