Look here, I said, "I may be altogether wrong. 
( ( Look here ) ( , ( I ( ( ( ( said , ) `` ) ( I ( may ( be ( altogether wrong ) ) ) ) ) . ) ) ) )
(ROOT (S (S (VP (VB Look) (ADVP (RB here)))) (, ,) (NP (PRP I)) (VP (VBD said) (, ,) (`` ``) (S (NP (PRP I)) (VP (MD may) (VP (VB be) (ADJP (RB altogether) (JJ wrong)))))) (. .)))
I couldn't think of any other explanation, however. 
( I ( ( ( could n't ) ( ( ( think ( of ( any ( other explanation ) ) ) ) , ) however ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD could) (RB n't) (VP (VB think) (PP (IN of) (NP (DT any) (JJ other) (NN explanation))) (, ,) (ADVP (RB however)))) (. .)))