A mysterious blast in a Tehran park was caused by a stun grenade, according to Iranian state television.
No one was injured in the July 10 explosion, which caused the capital's fire department to dispatch several units to the scene in the north of the city.
A stun grenade explodes with a loud noise but causes little damage.
A reporter for state television said the device had been set up to go off in the park near a hospital building.
Tehran Deputy Governor Hamid Reza Gudarzi, who is in charge of security issues, said earlier that the cause of the blast was being investigated.
"Just one explosion took place inside Mellat Park," he told the semiofficial Tasnim news agency.
Asked whether the incident was an attack, Gudarzi said: "We are investigating the dimensions and causes of the incident and we will provide information after we are sure."
A deputy police chief said an explosion was heard but that “no building was damaged or set on fire.”