On the morning of Dec. 14, 1971, NYPD undercover detective Frank Serpico put on his only suit. The son of an immigrant cobbler then stopped to get his good pair of shoes shined. The bearded 12-year police veteran was about to dismantle the department’s infamous “blue wall of silence,” exposing the NYPD’s systemic corruption across three hours of unprecedented testimony before the Knapp Commission and its probe of crooked cops.

A half-century later, the 85-year-old Brooklyn native remains a strident critic of police misconduct and a sounding board for fellow whistleblowers — while sharing his thoughts and concerns on a variety of topics with more than 5,000 Twitter followers. Serpico, who survived an on-duty gunshot to the face nine months before his testimony and death threats afterward, remains unsure how he’s lasted this long.