Why is there even such a thing as a “major US mosque?”
Why? Good question. An early Muslim Brotherhood operation in the USA was the Saar Foundaation: FYI
In the United States the Saar Foundation was connected to Al Taqwa, a shell company formerly based in Switzerland, where its leading figures included a notorious neo Nazi, Ahmed Huber. Al Taqwa was shut down after September 11th and its assets frozen by U.S. presidential order.
The Brotherhood, though damaged, shifted its accounts to “backup institutions” in the United States and elsewhere. Samir Salah, A SAAR board member and tenant in the Herndon, Virginia, office formerly administered a branch of the Muslim Brotherhood-dominated Al Taqwa operation in the Caribbean. Salah, administered a financial firm linked to the Saar operation and was a patron of the Dar al Hijra, a mosque in Falls Church, Virginia, noted for hard line Wahhabi/Salafi preaching.
From that day to this the Dar al Hijra has been considered one of the most radical of mosques in America.