Theodore Scardino, one of the firefighters "significantly wounded" in the shooting in Webster, New York, has spoken since treatment, say Strong Memorial Hospital staff.
Scardino is able to breathe on his own, but staff said both he and Joseph Hofsetter were "at risk" and being monitored for bleeding.
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Temperatures will reach around 28 Celsius (82F) at their highest.
A minute's silence will be observed at 11am on Thursday in remembrance of those who lost their lives in the Manchester Arena attack.
New research has found those who consumed two cups had a 35% reduced risk, and for those who drank five cups, the risk halved.