Who Broke Lincoln's thumb#5

In the fifth book of the Capital Mysteries—an early chapter book mystery series featuring fun facts and famous sites from Washington, D.C.—it's party time in Washington, D.C.! The president is planning a celebration at the Lincoln Memorial in honor of the man who sculpted the famous statue of Abraham Lincoln. But KC and Marshall notice one small problem - one of the thumbs has broken off the statue! Has someone stolen Lincoln's thumb? Can KC and Marshall find it before the celebration?