That is the current law, but the Old Post Office Building is taller than the U.S. Capitol (315 ft. vs. 288 ft.). It was built in 1899, 36 after the completion of the Capitol building. See this link.
In 1899, Congress passed the Heights of Building Act, saying that no building in Washington could be taller than the Capitor or other significant government buildings. A 1910 law further limits building heights to 20 feet more than the width of the street that the building fronts on. The situation with regard to architectural embellishments and the like is more complicated and somewhat controversial. See this Washington Post article.
Not that this has anything to do with Mohamed Atta, but I found this interesting and I didn't know it before I looked it up.