Complex sentence: The building was actually financed by Washington real-estate investors, who had heard rumors that some Congressmen were discussing relocation of the national capital in the aftermath of the burning, and wanted to prevent their land values from decreasing by keeping the government in Washington.
Split into simpler sentences: The building was actually financed by Washington real-estate investors, who had heard rumors that some members of Congress were considering relocation of the national capital in the aftermath of the burning. The investors wanted to prevent their land values from decreasing by keeping the government in Washington.