The city's fire department said only one of the victims was considered for "Level 1 Priority Care" and the other patients were hospitalized out of precaution.
The video above is the ABC13 Houston's 24/7 Streaming Channel.
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Their conditions were not immediately disclosed.
San Antonio Fire Department said a tree fell on a group, with the zoo adding that guests were the ones injured just after noon.
First-responding emergency personnel were on the scene right after the branch "unexpectedly" broke and fell, the zoo said in a statement.
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"San Antonio Zoo's Security/EMT team quickly responded and began treating guests. San Antonio Park Police, San Antonio Police and San Antonio Fire Department responded within minutes," the zoo's statement read, in part.
Aerial footage taken in the wake of the branch fall showed a large portion of a tree resting on the ground and covering part of a walking trail. The portion where the branch snapped was also seen from a news helicopter's camera.