David Jahnes, a 16-year department veteran, was responding to a call at the Chase Bank on North Broadway around 8:30 a.m. when he went into cardiac arrest, they said.
What was reported as a chemical spill turned out to be an overheated battery in the bank's alarm system, authorities said.
The building was temporarily evacuated, they said.
Jahnes joined the department in November 2001 and served as company captain from 2008 to 2011.
"Today, as well as the days, weeks, and months ahead please continue to remember Firefighter David Jahnes family, friends, and the Nyack Fire Department community in your thoughts and prayers," the department wrote in a Facebook. "Let us always remember and honor the memory of this courageous firefighter."
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