KEY WEST, Fla. (NBC) – Divers in the Florida Keys installed an American flag underwater to commemorate the 20th anniversary of 9/11.

Divers in the Florida Keys National Marine Sanctuary carried the giant 30-by-40-foot flag about 60 feet beneath the ocean’s surface and affixed it to the kingpost of the General Hoyt S. Vandenberg.

The historic military shipwreck was intentionally sunk to be an artificial reef.

The installation of the flag was followed by a flyover by two navy helicopters, observed by boaters including relatives and friends of people who died in the terrorist attacks and their aftermath.