A drone strike this week targeted a highly sensitive military site outside Tehran where Iran develops missile, nuclear and drone technology, according to three Iranians with knowledge of the attack and to a U.S. official.

The strike on Wednesday evening hit the site of the Parchin military complex, about 37 miles southeast of the capital, with quadcopter suicide drones, according to the Iranian sources, who were not authorized to speak publicly. The drones exploded into a building used by the Ministry of Defense for research on drone development, killing a young engineer who worked at the ministry and injuring another person, they said.

There was no immediate claim of responsibility, but the attack fit a pattern of past Israeli strikes on Iran and Lebanon in a covert campaign of hostility that has been going on for years. A statement from Iran’s Ministry of Defense indicated that it viewed this as an attack, not an accident.