According to the WSJ (from intel sources)

The US "has known about for years," about the facility, though it wasn't until more recently that they were confident that it was being used for uranium enrichment, said a U.S. Intelligence official.

The facility is hidden in an underground tunnel complex 30 km north of the city of Qom, Iran's Holy City. It is built to hold up to 3,000 centrifuges, though it's not clear that they're fully operational yet.

It's located at an IRGC base under the management of the Atomic Energy Association of Iran, however it's not clear that many of the people within that organization knew about the enrichment operation.