(AP) -- A firework that exploded when a 19-year-old unwittingly stepped on it Sunday in Central Park, seriously injuring his left foot, didn't appear to be designed to intentionally hurt people, police officials said.

The homemade gadget was like an "explosive experiment" that was probably designed to "make a large noise, maybe make a flash," concocted by someone with a basic understanding of chemistry, said Lt. Mark Torre, commanding officer of the New York Police Department's bomb squad.

There was no evidence that the explosion was related to terrorism and there were no specific, credible threats on New York over the July Fourth weekend, the authorities said.

The injured man, who police did not identify, was walking in the park with two friends when he stepped on a rock covering the explosive. He was undergoing surgery to his left foot at a hospital and was in stable condition, fire officials said.

Investigators don't believe the man who stepped on it or his friends are responsible for creating the explosive and officials asked that anyone with information about it contact the police.

"We've seen a lot of experimentation with homemade fireworks," Torre said, adding the Central Park explosive may have been designed to go off at an earlier time, even a day before, but for whatever reason did not.

The 11 a.m. blast on the east side of Central Park could be heard for blocks, leaving some with the belief that it was part of a July Fourth celebration.

Tourist John Murphy, visiting the city from Connecticut, stayed with the injured man until emergency responders arrived and placed a tunicate on his leg.

"His left leg was severely damaged, all bone and muscle," said Murphy.

Mayor Bill de Blasio sent out a tweet reminding people to stay safe around fireworks.

"Fireworks are fun -- but let's leave them to the professionals," the mayor tweeted.