More than five months after a mysterious substance exploded in Central Park, blowing off the lower part of a 19-year-old man’s left leg, many questions about the episode persist.

Law enforcement officials continue to believe the blast may have been the result of an amateur’s experiment with homemade explosives. But they have yet to determine why the explosive was in the park or to identify a suspect or make an arrest.

On Friday, the Bureau of Alcohol, Tobacco, Firearms and Explosives, which is investigating the explosion along with the New York Police Department, said it would offer a $12,500 reward for information that led to a suspect, bringing the total amount of reward money being offered by law enforcement agencies in connection with the blast to $25,000.

The authorities also asked for anyone to come forward who might have taken photographs or videos near the explosion site, around the entrance to the park at 60th Street and Fifth Avenue, in the days before the blast.