Kats said he tried to reassure his fellow passengers that it was a school project -- not a bomb -- but people scrambled for the exits nonetheless. The box he was holding had a small battery, wires and a motor.
"They were panicking, and I realized their fear," an apologetic Kats said.
He said he tried to disassemble the contraption on the platform even as he reassured riders, "Don't worry. This is my science project."
Kats was questioned by police and later released.
Kats is a computer engineering student at the New York City College of Technology.
"I'm very sorry for what happened," Kats said. "I hate to scare people. Next time, I'll be much more careful and keep my electronic projects at home."