Preliminary investigation suggests the explosion was an "experiment with fireworks or homemade explosives," NYPD Deputy Chief John O'Connell said at a news conference Sunday afternoon.
"We do not have any evidence of a constructed device or commercial-grade fireworks," he said. "There are no specific, credible threats directed at New York or the July Fourth celebrations."
It is not unusual for people to try and make homemade fireworks around the Fourth of July, O'Connell said, calling such activity "completely unadvisable."
Witnesses said the sound of the blast rippled through the park like a "cannon," scaring birds off trees. Some assumed it was a firework given the holiday weekend.
Nonetheless, the blast rattled New Yorkers, especially in the wake of recent acts of terror in the United States and elsewhere in the world.
The victim was identified as an 18-year-old tourist from northern Virginia. He was visiting the park with two friends, ages 18 and 20, O'Connell said.
Shortly before 11 a.m. the three young men jumped off a rock in the park, setting off the explosion that left one of them severely injured.
The victim was transported to Bellevue Hospital in serious but stable condition, where he was undergoing surgery. The victim and his two friends are not considered suspects, O'Connell said.
Evidence suggested the work of amateur hobbyists possibly experimenting with explosives during the July Fourth holiday, bomb squad commanding officer Lt. Mark Torre said.
He estimated that the explosive had been in the park for longer than a day.
Evidence suggested it was not meant to go off from someone stepping on it, Torre said. Someone may have tried to set it of earlier and left it behind when it didn't appear to work.
"Clearly, I would say that this is somebody that had knowledge of chemistry. But you don't have to be anything more than good with computers to get enough knowledge of chemistry to be able to create something of this type ... from readily available materials, unfortunately," he said.
"This is clearly explosive material. But there's nothing to indicate that this was an explosive device that was placed or put in this area with a specific intent to harm any individuals."
Explosive material from the scene was being tested at a bomb lab, a federal law enforcement official told CNN. Members of the FBI-led Joint Terrorism Task Force with expertise in explosives assisted, given the nature of incident, the official said.
Another law enforcement source told CNN the explosive material was contained in a plastic bag.
The park remained open except for the area where the incident occurred.