A large explosion occurred east of the Iranian capital of Tehran late Thursday night, according to multiple reports.

It remains unclear what caused the event.

According to the Al Arabiya network, the explosion took place at a military base east of Tehran.

New York Times reporter Farnaz Fassihi cited reports from the area saying that the site of the blast was the Parchin base where Iran has conducted nuclear activity in the past.

The BBC’s Persian language service reported, citing a witness familiar with the area of the explosion, that it most probably occurred in either a Revolutionary Guard base or another base belonging to the Iranian army.

The Iranian Mehr News Agency reported that a bright light was seen in the sky and is believed to be related to the explosion.

Iranian officials are investigating the cause of the explosion.

Iran has denied that the Parchin site is related to its nuclear program, though it did admit at one point to using Parchin to test exploding bridge wires, used as nuclear detonators.

In 2014, a mysterious explosion occurred at the site. While Iran denied that an explosion had occurred at the site, satellite evidence received later refuted the denial.