Beheld that which he did (theasamenoi ho epoiēsen). First aorist middle participle of theaomai and first aorist active indicative of poieō in the relative (ho) clause. They were eye-witnesses of all the details and did not depend on hearsay.
Believed on him (episteusan eis auton). Such a result had happened before (Joh 7:31), and all the more in the presence of this tremendous miracle which held many to Jesus (Joh 12:11, Joh 12:17).