While the evidence of Iranian involvement is strong, there are also signs that al Qaeda played some role, as yet unknown.
( ( While ( ( ( the evidence ) ( of ( Iranian involvement ) ) ) ( is strong ) ) ) ( , ( there ( ( ( are also ) ( signs ( that ( ( al Qaeda ) ( ( ( played ( some role ) ) , ) ( as ( yet unknown ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN While) (S (NP (NP (DT the) (NN evidence)) (PP (IN of) (NP (JJ Iranian) (NN involvement)))) (VP (VBZ is) (ADJP (JJ strong))))) (, ,) (NP (EX there)) (VP (VBP are) (ADVP (RB also)) (NP (NP (NNS signs)) (SBAR (IN that) (S (NP (NNP al) (NNP Qaeda)) (VP (VBD played) (NP (DT some) (NN role)) (, ,) (SBAR (IN as) (S (ADVP (RB yet)) (ADJP (JJ unknown))))))))) (. .)))
There is no evidence linking Iran to this.  
( There ( ( is ( ( no evidence ) ( ( linking Iran ) ( to this ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (NP (NP (DT no) (NN evidence)) (VP (VBG linking) (NP (NNP Iran)) (PP (TO to) (NP (DT this)))))) (. .)))